transportation through pipe lines of oil and other viscous ... · on the continent, the...

18 VICTORIAN INSTITUTE OF ENGINEERS.

PAPER TRANSPORTATION THROUGH PIPE LINES OF OIL AND

OTHER VISCOUS LIQUIDS.

By R. F. Kneale.

This outline deals expressly with the piping of oils, in keeping with the paper, "The Engineer's Part in the Petroleum Industry," viz., crude oils, refined lubricating and other residual oils, tar and bitumen.

Other industrial viscous liquids that can be included would be such as molasses, syrups, soap, varnishes, acid sludge, etc.

With bitumen and its compounded products, including other various viscous liquids, it is necessary to maintain a temperature.

The enormous supplies of crude oil produced during these days are transported in a major degree through pipe lines; gas and refined petroleum receiving particular attention.

Steel pipe lines are almost universally used, being made up in unit lengths, with special joints and couplings, as described elsewhere in this paper.

Friction losses, due to resistance of flow in regard to oil or viscous fluids in pipes, depend on the viscosity and density of the fluid and the velocity of the flow. Viscosity varies greatly with temperature, while also the density will change for the same reason.

Many complicated problems evolve relative to the determina-tion of friction and obvious pressure loss in pipe lines ; it is a subject upon which there is limited recorded data.

All real fluids possess a positive viscosity. The viscosity of a liquid decreases as the temperature increases; that of a gas increases in a similar order. A "perfect" or "ideal" fluid is hypothetical, and would have a viscosity of zero, and thus, no resistance to shear.

Viscosity is a measure of the resistance offered by a fluid to relative motion of its parts.

Adhesion between the pipe and the fluid is usually greater than the cohesion between the particles of the fluid.

When flow commences, a film of the fluid next to the pipe is retarded, and remains practically stationary, resulting in a condition of a fluid moving through a cylinder of the fluid.

Flow in pipes is of three kinds—streamline (at low velocities), unstable, and turbulent (at the higher velocities).

With viscous fluids in conjunction with pipes of small to medium diameters, streamline flow is considered, which results in

PIPE TRANSPORTATION OF VISCOUS LIQUIDS.

the least loss of head. In turbulent flow, there is greater loss of head and subsequent loss of pressure. Currents are set up in the body of a fluid when force is delivered. Within the pipe there is a force near the outer surface tending to retard; one on the inside tending to accelerate one tending to push the particle forward and force it toward the centre; and inertia tending to keep it moving in the same direction.

Pressure within the pipe can be resolved into two forces, one static and the other dynamic, depending on the velocity. The static force tending to produce flow is greater at the edge of the pipe area than at the centre.

To explain further the phenomenal characteristics, the follow-ing will be of assistance :

At low velocities, liquid in a filled pipe flows like an infinite number of thin, concentric moving cylinders of the fluid, the outer ones moving slower while each consecutive inner cylinder slides past its outer adjacent neighbour. This condition is known as "streamline flow." Under this flow, the fluid at the centre progresses at the greatest velocity, and the cylindrical film at the wall is almost stagnant. There is an effective trapping or retarding of the flow of the fluid near the wall of a pipe, due to the relative roughness compared with the characteristic thin cylindrical films of the fluid. Viscosity, or "drag," creates reactions accordingly which affect the body of the fluid.

In small pipes, the condition of the inner surface is of para-mount importance, but with pipes of, say, 6 in. to 12 in. diameter, the surface is much less important. •

Referring back to the previous mention of forces characteristic within the pipe with the fluid flowing, it can be explained that when these forces become sufficient to overcome the cohesion of the particles, a cross or irregular flow will be established, and the cylindrical sliding motions be superseded by an indetermin-able mixing of the fluid in the pipe. If the line pressure be gradually increased to produce this condition, the accelerated velocity would reach a point where the fluid whirls. The point at which this condition takes place is known as the critical velocity, and a state of "unstable flow."

With further added pressure under this condition, the velocity will not increase until a second condition is reached, due to rise of pressure, when the velocity and discharge will again increase proportionately according to pressure; the point then reached is called the "turbulent stage" of steady flow; the oil (or fluid) in this condition rolls through with a whirling, vortical, or circular motion. It is considered that this latter

19

20 VICTORIAN INSTITUTE_ OF ENGINEERS.

characterstic movement of flow, for lines carrying heated liquids, will retard to an extent, loss of heat per the pipe walls and any increase of viscosity of the liquid adjacent to the walls of the pipe. This is but an incidental consideration.

The stage where the characteristic change takes place has been designated the "critical velocity," and this varies accord-ing to conditions and in relation to the viscosity and specific gravity of the oil (or fluid) , also pipe diameter.

Flow is usually referred to as being streamline or viscous; unstable ; turbulent.

The critical velocity is indicated by a sudden change in the characteristics of flow. Friction head increases very rapidly when this velocity is reached, and this stage occurs earlier as the temperature increases; it is possible for the loss to be greater at, say, 60° than at 50° of temperature.

It is when a state of steady flow is reached, whether viscous or turbulent, that definite law prevails, and pumping effort expended accordingly.

The inside surface condition of a pipe is of more importance with streamline flow than with turbulent flow. As already described, the maximum velocity for streamline flow is at the centre, and for comparison can be reckoned at 90 per cent. greater than the average velocity within the pipe (nearly double) , while for turbulent flow the centre velocity is only 25 per cent. greater than the average velocity.

C oncerning Smooth' and Rough Pipes.

The terms "smooth" and "rough" are generally regarded as the physical characteristics of the pipe walls, but actually they must acquire a more dynamic significance when considering flow. A pipe can be smooth in the hydraulic sense, yet the resistance coefficient in turbulent flow is affected by the viscosity rather than the small changes in surface irregularities or roughness.

An identical pipe can be smooth under one set of flow conditions and rough under another. A pipe is "rough" if its friction coefficient is independent of the viscosity.

It is the difference between rough and smooth in the above dynamic sense that determines the whole range of flow conditions, so important in engineering undertakings.

As already intimated, the inside surface condition of the pipe line as regards "smooth" and "rough" is more important with streamline or viscous flow than in turbulent flow. The condition

PIPE TRANSPORTATION OF VISCOUS LIQUIDS. 21

characteristic of the latter flow tending to differentiate it from that of streamline and as being affected is as follows :-

In fully developed turbulent flow there is surrounding the central core of the fluid a cylindrical-like laminar film in motion, adjacent to the pipe wall. Data are scarce pertaining to the flow motion of this particular layer, but it is assumed to have a thickness dependent on time and position, etc. Apparently this laminar film sheds vortices, which actually form the turbulent core. . The average bulk of the core is speculative, as the laminar film thickness can only be approximated. When it is thin the linear tractive force will attain a more constant movement.

The viscosity of the fluid determines the distribution of flow movement, and for this reason it is obvious that if the fluid temperature is raised the surface velocity is increased.

In the majority of practical oil pumping problems, that of viscous or streamline flow is mostly considered.

Viscosity being greatly affected by temperature does not seem to follow any particular law. Oils having similar viscosities at one temperature may have definitely varying viscosities at another temperature.

At temperatures between 40° Fah. and 70° Fah. (4.4° C.-21 ° C.) , the viscosity of all oils changes very rapidly, while at temperatures of 200° Fah. (93.3° C.) and upwards, the change of viscosity with temperature is relatively small.

Viscosities are determined with the aid of a viscometer ; the three mostly used being

In England, the "Redwood" instrument. In America, the "Saybolt" instrument. On the Continent, the "Engler" instrument.

This article is mostly concerned with the "Redwood" instru-ment. There are two types, the No. 1 and No. 2 instrument, the former being used for the less viscous oils, and the latter for the very heavy and viscous oils.

To convert approximately from Redwood Viscometer No. 2 reading to Redwood Viscometer No. 1 reading, multiply by ten, provided the No. 2 readings are not less than 100.

In conjunction with viscosity is specific gravity, which in a lesser but nevertheless definite degree is affected by temperature. Physical and chemical characteristics determine this measure and basis.

On the American Petroleum Institute Beaumé scale, which is used chiefly in America, water has a gravity of ten, after which the reading increases with decrease of specific gravity.


For practical operations in refining petroleum, the specific gravity is dealt with in terms of the Beaumé scale, which in actual reading conveys no idea of the specific gravity.

A useful table of conversions is as follows, based on a tempera-ture of 60° F. For other temperatures adjust the specific gravity shown by subtracting .0004 for each degree above 60° F., and adding .0004 for each degree below 60° F.

Specific Beaumé Specific Beaumé Gravity Gravity Gravity Gravity 1.0000 10.0 .9280 21.0 •

.9956 10.5 .9220 22.0

.9895 11.5 .9165 23.0

.9825 12.5 .9105 24.0

.9755 13.5 .8990 26.0

.9685 14.5 .8870 28.0

.9655 15.0 .8755 30.0

.9595 16.0 .8650 32.0

.9530 17.0 .8545 34.0

.9465 18.0 .8440 36.0

.9400 19.0 .8345 38.0

.9340 20.0 .8250 40.0

Readings are referred to as degrees Beaumé.

The simplest method of determining the specific gravity is by the ordinary hydrometer, calibrated to give direct readings. Due to the natural expansion of oil on being heated, its specific gravity will decrease; it is, therefore, usual to convert all specific gravity readings to a standard temperature, for example, 15° C. or 60° F.

Dealing further with vicosity, it can be described as resistance to internal movement of minute particles of the mass, one upon the other.

"Absolute" viscosity is technically the force in dynes neces-sary to move a surface of one square centimetre past an equal parallel surface of one centimetre distant with a velocity of one centimetre per second, the space between being filled with the fluid whose absolute viscosity is sought. This viscosity is expressed in dynes per square centimetre, its unit being the poise, equal to one dyne per square centimetre. The dyne is a unit of force, which, acting on a gram for a second, produces a velocity of a centimetre per second.

It is usual, and for the purpose of further reference in this paper, to deal with absolute viscosity in terms of equivalent English units; thus there is a coefficient representing "foot pound, second, units."


"Kinematic" viscosity is the absolute . viscosity divided by the specific gravity.

Many engineering problems become evident when, consider-ing the transportation of oil through pipe lines, whether it be crude or refined gasoline. The system has developed to such an extent that it is estimated that in the United States of America alone more than 112,000 miles of pipe lines, forming definite systems, are functioning. Included in this estimate are about 60,000 miles of main trunk lines and 53,000 miles of gathering lines; these figures, so formidable, are irrespective of other great lines and systems in other parts of the world. There is continual increase as time advances.

It is important that the characteristics of the product to be pumped be studied, and engineering application made accordingly.

As an aid to the solution of problems arising from flow and pumping, it will be an advantage to present a few figures and formulae sufficient to facilitate investigation and recom-mendation.

Having previously discussed the characteristics of flows and differentiated between viscous, or streamline, and unstable flow, the critical velocity and turbulent flow, it is proposed to consider from the engineer 's standpoint the conditions applied in the determinations arrived at in pipe line problems; these hinge around either the streamline or turbulent flow, comprising two conditions.

A summary of deduced formulae and coefficients, etc., for useful application, is as follows :-

For times exceeding 200 seconds Redwood No. 1.- Absolute Viscosity in

(English Units-Foot pound = Time in secs. Redwood No. 1 x seconds) Specific Gravity

5720 For times below 200 seconds Redwood No. 1, the following

table will suffice :- Time in Spec. Absolute Spec. Absolute Spec. Absolute Spec. Absolute Secs. Gravity Vise. Gravity Vise. Gravity Vise. Gravity Gravity 50 1.0 .0067 .9 .0061 .8 . .0053 .7 .0047 75 1.0 .0115 .9 .0103 .8 .0090 .7 .0079

100 1,0 .0162 .9 .0146 .8 .0128 .7 .0112 125 1.0 .0209 .9 .0188 .8 .0166 .7 .0145 150 1.0 .0256 .9 .0230 .8 .0204 .7 .0178 175 1.0 .0303 .9 .0272 .8 T .0241 .7 .0211 200 1.0 .0350 .9 .0314 .8 .0278 .7 .0243


Viscous or Streamline Flow.

The coefficient "A," as given below, enables friction loss in lbs. per sq. inch for each 100 feet of pipe to be calculated, using the following formula, for straight pipe only :—

P NxAxV 52.8

H _NXAXV 22.915

where—P = Loss in lbs. per square inch for 100 feet of pipe line.

H = Loss of head per 100 feet of pipe line. N = Absolute viscosity in foot pound seconds. V = Velocity in feet per second. A = Coefficient.

Values of "A" Relative to Various Pipe Diameters.

Diameter A Diameter A 1" 5280 6" 147 11" 2590 7„ 108 2" 1460 8" 82.5 22" 932 9„ 64.3 3" 587 10" 52.8 31" 430 12" 36.5 4" 330 14" 27 5" 212 15" 22.5

Turbulent Flow.

Prior to the critical stage, there is a condition of unstable flow. Following the characteristic change at the critical stage cornes the turbulent flow.

Turbulent flow is known to occur when the value expressed as-

DVW exceeds 2500 N.

where—D = Diameter of pipe in feet. V = Velocity in feet per second. W = Density in lbs. per cub. foot. N = Absolute viscosity in foot pound seconds units.


The coefficient f and constant C as given below enables friction loss in lbs. per sq. inch for each 100 feet of pipe to be calculated, using the following formula. for straight pipe only.

CfWV2

Cf WV2 H — _

22.915

where—P = Loss in lbs. per square inch for 100 feet of pipe line.

H = Loss of head in feet per 100 feet of pipe line. C = Constant varying with pipe dia. f = Friction coefficient. W = Density in lbs. per cub. foot. V — Velocity in feet per second.

Values of C and f—

Dia. of pipe— 11 2" 1 3" 1 4" 1 6" 1 8" 1 10" 1 12"

C = 11 3.416 12.277 11.708 11.138 1 .854 1 .683 1 .569

P =52.8

_

5

8 8

DVW f DVW f DVW f DVW f N N N N

2,000 9

2,200 ~ ~

2,400 v x

A .032 .0291 .0266

.0442

.0426

.0400

5,0001 6,000 7,000

8,000 9,000

10,000

.03821

.0364

.035

.034

.033

.032

20,000 .0264

30,0001 .0238

40,000 .0219

50,000 .0208

60,000 .02

70,000 .0195

80,000 90,000

100,000

150,000 201,000 250,000

.019

.018

.018

.016

.015

.015

2,500 G

3,000 -1 ó 1,000

A wv

The detailed data will prove useful for ordinary problems of transportation through pipe lines of oil and viscous liquids, while as an additional interest a few facts regarding oil and further application of simple computations will be an advantage.

Viscosity characteristics of various grades of oil, recorded on the Redwood Viscometer No. 1 are as follow :—

70° Fah.

140° Fah.

45-60 secs.

60-95 secs.

Motor oils and heavy spindle oils

Light machinery oils Machinery oils Steam cylinder oils

150 secs to 300 secs.

300-800 secs. 800-5000 secs.

95-350 secs. 550-1200 secs.

26 VICTORIAN INSTITUTE OF ENGINEERS

Anti-freezing Oil.—This oil is light in colour, and very clear. The absolute viscosity is not more than 1.06 poise at 32° Fah. (0° Cent.), nor less than 0.10 poise at 100° Fah. (37.8° Cent.). The oil; according to specification, must not cease to flow when exposed for 20 minutes to a temperature of minus 50° Fah. (minus 45.5° Cent.). This oil is used for the lubrication of controls, machine guns, windmill pumps, watches, etc., on aircraft.

Relative to ordinary fuel oil, a general formula for service pipes, etc., is as follows

H = 300,000 d4 where—H = Loss of head in feet.

R = Viscosity secs. Redwood No. 1. L = Length of pipe in feet. Q = Quantity in gallons per hour. d = Diameter of the pipe in inches.

Viscosity for fuel oil can be taken as 125 secs.

Where the kinematic viscosity (Vk) of oil in poise units is known, the formula is : '

H=LQ (Vk) 750 d4

This formula applies with reasonable accuracy for deliveries up to Q = 3160 (Vk) d.

Above this, the oil flow becomes turbulent instead of viscous, and the applicable formula is :

Q2 L 2,400,000 d5

L Q R

H=

PIPE TRANSPORTATIOI\ OF VISCOUS LIQUIDS. 27

TABLE OF VISCOMETER READINGS 8c CORRESPONDING KINEMATIC VISCOSITIES.

P Vk P Vk P Vk P Vk P Vk P Vk

26-7 •010 552 -109 85 6 -/97 114-6 -274 1437 -348 1773 •432

27.0 •011 55.7 •1 11 86-2 •199 115.4 -276 144-1 349 179•o - 436 27-8 -013 56-3 •1/3 867 •200 /15 -9 •277 1448 -35/ 1805 -440 28-2 -015 573 •116 87-4 -202 1166 -279 /456 -353 181- 9 •443 28-7 -016 57.9 •118 88-2 •204 /17-3 •280 /46-0 -354 183.4 •446 29-4 -019 58-5 •120 88- 9 •206 /178 -282 /46.6 -356 184- 8 •450 30- 1 -020 '59.2 -122 89-4 •207 /18-6 •284 147-2 •357 /867 -455 30-7 -022 59-8 -124 9o-1 -209 119.1 - 285 /48-0 -359 1885 .460 31 • 3 -024 60-5 -126 907 •211 119•8 •287 148.7 •361 /89.8 •463 31•9 -026 61• 1 •128 91-6 -2/3 120-3 •288 149-2 -362 191.3 -467 32-5 -028 61.8 •130 91-9 '2 /4 /20-9 -290 150-o -364 192-8 •470 33 -0 -03 62-5 •132 92-6 •216 12/ -3 •291 /50-5 •365 194-6 •474- 33•6 •032 63.2 •134 93-3 -218 /22-0 -293 151•3 -367 /96-2 '478 34-2 '034 63-8 •136 94-0 •220 122-8 •295 152.0 -368 /978 •482 35.0 -037 64•5 •138 94-4 •221 123.2 •296 /52-5 •370 /99 -3 -485

35-4 -039 651 •140 95-1 -223 /24•0 -298 152 - 9 -37/ 2008 -490 36-0 •041 65.7 •142 958 -225 124-5 •299 153-7 -373 20/•3 -49-4 36- 8 -044 66-4 -144 96-6 -227 /25-2 -30/ /54- 4 -375 203-8 •498 37-4 -046 67-I •146 97-4 -229 /25-9 .303 /54•9 -376 205-4 -502

38•0 •048 67-8 -148 978 •230 /26.4 -304 /55- 7 -378 207.0 -506 3&5 -050 68.4 -150 983 •231 127.2 -306 /562 -379 208-6 5/0 39-3 -053 69-1 •152 98-8 -233 /28.0 -308 /56 .8 -381 210-3 -.5/3 39-7 •055 69-5 -153 99 5 -235 /28.4 -309 /57-3 •382 2/2-o -5/7 •40.3 •057 70-2 •155 /00 -1 -237 /29-2 -311 1581 -384 2/3.8 •52/ 41• 1 -060 70-9 •157 /007 -238 129-9 -3/3 /58.9 •386 2/5•3 -52 5 41•9 -063 7/•5 •159 /01.2 -239 /304 - 314 /59.4 •387 2/7•0 •530 42-4 -065- 722 -161 lor -9 -241 /3/•1 •3/6 /60 -0 •389 2/84 533 42.9 -067 72.9 -/63 /O2• 7 •243 13/-6 -3/7 1605 '390 219-8 -536

435 -069 73-4 -165 103.5 -245 132•1 •3/9 16/.3 '392 2213 •5-40 44-3 -072 73-9 -166 /04 -2 -247 1327 •320 /6 /-8 •393 .223•0 -544 45-2 -075 74-7 •168 /04-8 -249 /33-0 •321 162-5 •395 2 24.6 -548 457 '077 75.4 -170 /05'2 •250 /33.8 •323 /63 - / 397 226.4 -552 46-3 •079 76.1 •172 /05- 9 •252 134.6 -325 1637 398 .227.9 •556 46-8 •08I 76-9 -174 /06-4 •253 /35 - 2 -327 /64 5 •40 229-4 •560 47-4 •083' 77.4 -175 /07.2 -255 /358 -328 /65-0 -401 2309 -564 48-3 -085 78-0 -177 /077 - 257 /36.3 -330 /65-8 •4o3 232-4 .568 48-6 -087 78-6 -179 /08.2 -258 /37•0 331 /664 -404 233-9 -572 49-1 -089 79-3 •181 /09-1 .260 /37-8 -333 167•0 - 406 235•4 •575 50.0 -092 79-8 -182 /09.6 •26/ 138 -6 -335 /67-5 -407 2368 -579 50-7 •094 80 5 •184 /10.3 -263 /39.4 -337 /68-2 -409 238-4 •583 51-2 -096 81-3 •186 /11.1 -264 / 39•8 -338 /68 -6 -4/0 239-6 •586 52-1 -099 82-0 -188 /11•4 -266 140-6 -340 /69 •6 -4/2 241.8 '590 58 •101 828 -190 /12.2 -268 14/•0 •34/ 1708 •416 2432 -594 53•4 •103 836 •192 //3-0 . 270 14/-6 •343 /72.6 -420 2448 •598

53-9 •105 84-2 •193 /13-8 -272 142.1 '344 /74•3 •424- 245 5 •600

•l07 :4 9 - /9 /4•/ - 73 14 -9 -346 /75.8 -428


The crude oils of Mexico are a heavy crude, yielding little gasolene, but a very large percentage of asphaltic bitumen.

Chemical treatment is necessary to make saleable the products from Mexican and Egyptian crudes.

The crude oil of Tarakan, in East Borneo, is a natural liquid fuel. Borneo and Sumatra crude oils need no refining.

The viscous flow for this particular oil can be calculated as follows :—

H = .0066875 d2 where H — Loss of head in feet per 100 ft. of pipe.

V — Velocity in feet per second. P = Density in lbs. per cubic foot. T — Viscosity measured with Redwood No. 2. d =: Diameter of pipe in inches.

To convert approx. from Redwood No. 1 readings to No. 2, divide by ten.

Valves, Bends, etc.

Equivalent Length of Straight Pipe in Terms of Diameters.

TVP

Unit Viscous or Streamline Flow Turbulent Flow

Angle Valve Gate Valve Globe Valve Right Angle Bend Elbow Tee

20 to 40 times 6 to 10 „

25 to 50 2 to 3 „ 6 to 8 „

40 to 60 „

100 to 120 times 12 „

125 to 150 „ 4 to 6 „

30 to 40 „ 150 to 175 „

The figures given are to a certain degree sufficient to enable an investigation into oil line requirements pertaining to pressure for overcoming friction; this, coupled with consideration due to any static head in the line and converted to equivalent lbs. pressure, will introduce the necessary pumping capacities.

The initial pressures in pipe lines of substantial length is somewhat arbitrary, and determined and dependent upon the line friction and accumulated static head, which in turn depends absolutely upon the character of the oil. Pump pressures range from 500 to 1000 lbs. per sq. inch ; the engineer designing according to conditions and characteristics already referred to.

For general installation work, pumps are usually of the reciprocating, duplex, double-acting type. These direct steam type of pumps have the advantage of speed flexibility, being controlled by the main steam valve, and are practically foolproof. The principal drawback is low efficiency and space occupied.


Another type considered in regard to oil and viscous liquids is the rotary pump, which takes many forms, and generally runs at a relatively low speed. Special types of rotary pumps with positive displacement may be used for pumping oils of very high viscosity, and are outstanding for this special duty.

The pumps mentioned and their application to any particular service depend on the nature of the oil or fluid to be pumped, together with suction and pressure conditions ; the prime motive power, whether electric or steam, and personnel for supervision, etc.

The power required to pump oil depends on the volume handled and necessary pressure, irrespective of the oil being viscous or very fluid ; the adjustment will be in the size of pipes.

Gravity and field lines have low pressures generally, and require care in their layout to avoid pockets that would cause them to become "air bound." In lines operating under high pressures, it is not so necessary to trap or release the air or gas. The oil flowing over the high point in the line, though diminished in volume by the compressed air or gas, will soon absorb the gaseous material and eliminate the obstruction. If water was being conveyed through the pipes, entrained air would greatly reduce the carrying capacity, and in such cases extensive use of air release valves is advocated. The aspect is different when dealing with volatile oil.

The contour of a long pipe line is a matter of some importance. No undue loss of capacity will develop so long as the contour of the pipe does not anywhere along the line rise above the hydraulic mean gradient. If on the other hand, when passing over elevated ground, any point in the line is above the gradient (not necessarily higher than the initial head), that portion to and from such point will be in a state of partial vacuum, and gas will be given out and accumulate at the summit, remaining more or less indefinitely in the pipe in the upper portion.

When a long pipe line is composed of different sizes of pipes, and is laid over ground of uneven contour, it is wise to investi-gate the gradient pertaining to each specific size and relative and included high points.

It is necessary, in certain circumstances, and relative to the magnitude of the project, to boost the oil along, and for this reason intermediate pumping stations are established.

The spacing of these stations is determined by practicable maximum working pressure, which must overcome the static head and line friction.


Maximum and minimum spacing usually ranges between twelve and sixty miles, although if there be a natural static head at the source of supply, the flow is assisted naturally, and average spacing is longer. Conveying light oils with normal static heads would probably require pumping in intervals of 40 miles. Very heavy, viscous oils would need pumping about every fifteen miles.

At the pumping stations it is customary to have storage tanks ; while one is filling, the pumps pull from the other. The whole project under consideration is evolved by the engineer, and in his survey and subsequent computations considers prevailing conditions, nature of soil and temperatures encountered, coupled with supplies and demands. It can be said that the engineer calculates, predetermines, designs, and applies accordingly.

Referring to the great Iraq line of 1150 miles length, there are twelve pumping stations included along the enormous length, ranging from 50 to 150 miles apart.

Seasonal temperature variations affect the oil flow in respect to viscosity, which causes initial pumping pressure fluctuations and capacity deficiencies. It can be assumed that more oil can be pumped in summer than winter.

Pumping equipment important and relative . to oil field produc-tion includes the Diesel engine driven centrifugal pump; operat-ing speeds ranging from 3500 to 3600 r.p.m., giving maximum mechanical efficiency. A characteristic of the centrifugal pump is its practical constancy over a speed of from 70 to 100 per cent. of its full rated speed.

Steel pipe of 70,000 pounds per sq. inch tensile strength is usual, and a safety factor of four adhered to. Pipes ranging from 6 in. to 18 in. diameters and 4 in. wall thickness will permit of initial working pressures between 500 pounds to 1000 pounds per sq. inch.

As a sequel to the reading of this paper, certain fundamental laws of fluid friction should be remembered, namely, according to Froude :-

1. The friction varies directly as the extent of wetted surface. 2. The friction varies directly as the roughness of the surface. 3. The friction varies as the square of the velocity. 4. The friction is independent of the pressure. 5. The friction varies directly as the density and viscosity of

the fluid.

Resistance is very small at slow speeds, and below the critical stage ; depending, as has been previously described, upon the


nature of the liquid and its temperature ; this resistance is proportional to the speed. Above the critical stage, the resistance is proportional to some power, approximately the square of the speed.

Resistance is diminished by an increase in temperature of the liquid, when this reduces the viscosity.

The chart shown is a means of carrying out a preliminary investigation for comparison of water flow. Coupled with the brief treatise given on oil flow, the results will provide interesting data for the engineer concerned.

Note.—The absolute viscosity of clear water at 68° Fah. (20° Cent.) is .01208 poise, or say approximately one centipoise, representing a Redwood No. 1 viscometer reading of about 26 seconds.

In converting static lift in feet to lbs. per sq. inch pressure, it should be noted that the water constant is .434, or feet head X .434 = lbs. per sq. inch.

Regarding oils and other viscous fluids, the constant will depend upon the density, which is affected also by viscosity: but it is sufficiently accurate to multiply the water constant by the specific gravity of the fluid in question, viz., for a specific gravity of .95, the constant would be .434 X .95 = .4223.

Two useful points to remember when considering the trans-portation of a liquid through piping, based on water flow, are :

(a) The discharge of a pipe in cubic feet per second equals the area of the pipe in square feet multiplied by the velocity of flow in feet per second.

(b) The velocity of flow in a pipe varies approximately as the cube root of the square of the diameter.

6

z O W N 4

3

2

9000 6000 7000

6000

5000

72 66 - 60 -

54-

48 - 4000

4

100

90

80

70

3- I

~

2-

42 -

36,

30 — 2000

28 26

w F 24-

22-.

20-H

W 000 a

900

800 ~ Z E

700 JO ~

600 O 2

500 ?

400

300

~

2

10

9 8 3000

7

60

VE

LO

CIT

Y IN

FE

ET

PE

R

0 z 8 J)

I8

16 -'

14

12 -

10-

9-

6 -'

7-.

.6 ~. 5 -1 200 ~

H

50 1.5

32 VIC7TOItIAN INST ITUTE OF ENGINEERS.

FOR CALCULATING PIPE SIZES , DISCHARGE, AND LOSS OF HEAD IN

PIPES FOR WATER.

(A PRELIMINARY INVESTIGATION OF THIS CHART WILL SERVE AS A COMPARISON WITH COMPUTATIONS FOR OIL AS OUTLINED N THIS PAPER)

NOTE:- THE ABSOLUTE VISCOSITY OF CLEAR WATER AT 68° FAH. (20° CENT.) BEING •01028 POISE OR SAY APPROXIMATELY ONE CENTIPOISE, REPRESENTING A REDWOOD NQI

VISCOMETER READING OF ABOUT 26 SECS.

PLACE A STRAIGHT EDGE (AS SUGGESTED BY BROKEN LINE) ON SCALES AT THE POINTS FOR ANY TWO KNOJVN QUANTITES, 8 TIE UNKNOWN QUANTITIES WILL BE INDICATED AT THE INTERSECTION OF TI-E STRAE9-IT EDGE WITH OTHER SCALES.

FOR PIPES OF AVERAGE SMOOTH CLEAN BORE.


Shock Pressures.

Conditions arising in the functioning of transportation oil lines present varying difficulties. One problem among the many is recognised as shock pressure, or what is commonly called "water hammer," due to generally considering water. There has been a dearth of data in regard to this particular problem. Shock pressure in pipes should never be neglected by engineers, particularly in long lines, otherwise serious damage will result. The problem is very involved from a mathematical viewpoint, and to such an extent that consulting engineers generally resort to purely arbitrary rules in finding the pressure rise.

Actual shock pressure is usual in long pipe lines and where flow is stopped abruptly by operation of valves closing quickly. Severe shocks are attendant, particularly where pumping velocities are high and pumps working at full capacity, which is so characteristic in the functioning of modern equipment. Severe bends, causing quick changes in direction of flow, or different diameters of pipes in the line, all tend to develop shock waves, and even amplify the same. Fluid flowing through a pipe line under regular, constant conditions can be greatly affected if instant closure of a valve against a substantial potential head occurs, and brings to bear the following :—Kinetic energy is liberated when the liquid comes to rest, and is converted into shock pressure, which causes a repercussion or pressure wave through the pipe to the intake and closed end.

For practical propositions, allowance for shock or hammer could be based on the following data and formulae

The fluid within and the pipe wall will be considered elastic as distinct from the rigid theory, actually modifying the pressures when computed relative to shock, in many cases. The bulk modulus of elasticity of the fluid can be represented by "k" lb. per sq. ft., while E is the modulus of elasticity of the pipe wall material.

Some approximate values of É are :-

Steel Plate .. .. .. .. .. .. 0.0097 (say 0.01) Cast Iron .. .. .. .. .. .. .. 0.020-0.022 Concrete .. .. .. .. .. .. .. .. .. .. 0.10 Wood .. .. .. .. .. .. .. .. .. .. 0.20

The ordinary pipe velocity rarely exceeds 15 feet per second, whilst the pressure wave (responsible for shock) always exceeds 2000 feet per second, and seldom is less than 3000 feet per second. Calculated values of this velocity under actual operating


conditions vary from 2500 to 4500 feet per second, being relative to the liquid and the pipe under consideration.

As previously stated, the maximum shock is developed through-out the line when the valve is closed instantly.

It is interesting to note that experiments and established data give the velocity of sound in water of medium temperature as 4665 feet per second, which, according to established formulae, approximates "a" (as later designated) if the walls of the pipe were assumed rigid. Actually "a" in the following formula is less than 4665 feet per second on account of the influence of the elasticity of the pipe wall and the ratio of its diameter and thickness, etc. Therefore, considering the elastic theory—

4665 a =

\I1+Xe

P = ad g

where—a = Velocity of shock pressure wave, feet per sec. k = Bulk modulus of elasticity of liquid. E = Modulus of elasticity of pipe wall material. e = Thickness of pipe wall in feet. d = Density of liquid, lbs. per cub. foot. D = Internal diameter of pipe in feet. P = Shock pressure in excess of normal pressure in

lbs. per sq. ft. V = Velocity extinguished by the closure of the

valve (ordinary constant velocity in the pipe) . g = Gravitational constant = 32.2 ft. per sec.

Maximum shock pressure is developed when the closing time of the valve is less than that for a cycle of the pressure wave in the line ; this critical time is represented thus—

T = 2N a

where—T = Time for a cycle of the pressure wave, in seconds. N = Length of pipe from valve to intake, in feet.

If the time of valve closing is greater than T, the maximum shock pressure will not develop. Shock pressure tends to decrease with a longer valve closing period.

Assuming the velocity of the pressure wave as a constant for a definite line and a given liquid, having referred previously to the elastic theory, due to characteristics of the line and liquid, it can be considered that the velocity of the pressure wave is a

o ![',

ti,-3 R


function of the properties of the liquid and of the pipe itself, independent of the length of the line, and that shock pressure is directly proportional to the velocity extinguished by the closing of the valve, etc.

Protection from Shock Pressure.—This is often afforded in pumping systems by the introduction of air vessels, which, to be effective, must be located near the source of the pressure wave. A sufficient volume of air must be available within, otherwise, if inadequate, a tendency to amplification of the pressure wave could develop. There are various other methods adopted, one being surge relief valves, used for relieving shock pressure. The problem is one for special consideration, and enters into pumping schemes.

2

f". Construction and Laying of Pipe Lines.

Thousands of miles of pipe lines for transmission of oil, gas, and water have been field welded by the electric arc process. These lines, ranging from small to large diameters, are laid across country under varied conditions ; rivers, valleys, moun- ..-.

_ tains, deserts, highways, and railways are crossed. Oil and gas lines of arc-welded construction have either plain

or bell-mouthed (faucet-like) ends forming the butt joints. In lines of 14 in. diameter and smaller, a liner or backing

ring inside the pipe beneath the joint is not required. It is a plain, straight-type, butt joint, and is used almost exclusively in oil pipe lines; it is economical, and obviates the cost of bell-mouthed ends. A liner or backing ring is used for over 14 inches diameter.

In laying oil or gas lines, the pipe is lined up in sections. The number of pipe lengths per section varies according to the

}~ length and diameter of the pipe. As a preliminary, the joints are generally tack-welded when

the pipes are lined up. After a section is completely welded, it is placed over or in the trench and welded into line. Part of the section is left out on skids until such time when contraction is maximum, when it is put into the ditch and covered. Expansion and contraction are provided for according to circumstances and conditions. Contour of ground and large bends-or sweeps all tend to compensate for any variations thus caused.

Expansion joints are used in lines laid above or on ground surface, with suitable roller or other type intermediate supports, giving free movement.


Anchorages are also provided in locations determined by the engineer. The orthodox type of expansion joint is telescopic, with glands and special packing; this in contradistinction to the special expansion bends on steam and high temperature lines, where expansion becomes paramount.

The advantages of arc-welded piping systems cannot be over estimated, and are becoming more obvious as time advances.

The great oil route of 1150 miles of pipe line from Northern Iraq to the Mediterranean coast mentioned in a previous paper is probably the most outstanding achievement of its kind in the world. Mention has been made of the welded joints and lengths of pipe 400 feet or more being lowered into the trench, the combined weight of pipe laid being approximately 120,000 tons.

For joining the pipes, the ends were expanded to accommodate a sleeve which fitted into the ends of the adjacent pipe.

Lengths of pipe 40 feet long were welded together in units of about ten or eleven. With such lengths of metal and varying temperatures and seasons, it was necessary to allow for expan-sion and contraction. At points seventy yards apart the pipe was supported on skids, and slightly above the level of the trench, and allowed to sag. After removing the skids, the pipe was forced down to its determined level, actually producing compression.

The possibility of the pipe contracting sufficiently to pull it apart was thus obviated. The earth filling is most important in the modifying of probable temperature variations.

Where light or non-viscous oil is to be conveyed, the pipe is laid directly on the ground, thus facilitating frequent and regular inspection and easy access for repair. If the oil to be conveyed is viscous and needs warm temperatures, the line is buried under the soil for about 18 inches. Protective covering for the pipe line is always given ample consideration.

Great precautions are always taken to protect and guard oil pipe lines. The nature of the products being handled vary, and in a greater or lesser degree call for caution against many developments that would result in dangerous and devastating occurrences. Apart from the transporting of the huge quantities of crude from the oil fields to the refinery, there are the great systems of distribution. Bulk cargoes from tankers are pumped through lines to storage centres, etc. Kerosene, in particular, is a dangerous product to handle, and recent investigations have established beyond doubt that the risk of forming a dangerous

PIPE TRANSPORTATION OF VISCOUS LIQUIDS.

The subject is one for special and separate consideration.

static electrical charge when pumping is much greater when the kerosene in the line is contaminated with water, air, etc. The risk of a static discharge when the line is free from such and the product uncontaminated is eliminated.

It is considered that during the first half-hour of pumping the velocity of flow should be limited to three feet per second maximum, say one metre.

After this preliminary period, there is no restriction on the rate of pumping. This procedure should be adopted always when the line has previously been cleared with water.

In concluding this short paper, it is well to mention the paramount importance of the economic life of long pipe lines, which demands due consideration of all means whereby the same is prolonged.

Corrosion plays the greatest part in the depreciation and shortening of the useful life of these expensive systems.

Special coatings of various compounds and wrappings have been resorted to, and applied with marked success. These protective coating compounds are derived basically from coal-tar, having properties, in effect, that will not crack, will not oxidise or disintegrate, are not affected by soil acids or alkalis, are impervious to water, and are able to resist electro-chemical action.


DISCUSSION

MR. LEONARD asked Mr. Kneale if he could state the bore of the Iraq line, and if it varied. Also he would like to know the number of pumping stations, and if crude or refined oil was transported.

MR. KNEALE replied (in writing) as follows :—There are two lines, one of which terminates at Haifa, in Palestine, and the other at Tripoli. Both run together for the first 156 miles to Haditha. The lines are 530 and 620 miles in length respectively, and the diameters 10 in., 8 in., and 6 in., all joints being welded. Twelve pumping stations are situated at distances from 50 to 150 miles apart. The pumps are of reciprocating type, direct coupled to 500 horse power Diesel engines. The line rises to 2600 feet above sea level, and drops to 850 feet below at one spot. Four million tons of refined oil are pumped annually. Seven million gallons of crude per day is pumped to the refining plant at Kirkuk, Iraq, which is 85 miles south-west of Mosul.

MR. ROBINSON asked if Mr. Kneale could state the rate or character of incrustation of oil lines.

MR. KNEALE replied that oil lines generally maintained a smooth, clear bore ; but if the product was changed, water was generally used as an intermediary for clearing the line. Chemical action or contamination may settle out some of the more solid contents of the oil, especially in the case of heavy black oils. Where fresh water was difficult to obtain, and sea water was used, this introduced definite possibilities of incrustation.

The PRESIDENT (Mr. Morris) mentioned that Aìistralian firms had probably done more pioneering in the supply of light, strong pipes for high pressures than those of any other country during the past 30 years. He said that great progress had been made by the application of suitable internal and external coatings, which eliminated much internal friction and external corrosion. Glazed pottery pipes had been much improved in strength of late years, whilst spun reinforced concrete pipes, by being rattled during spinning, acquired a very smooth interior finish.

MR. POLLOCK proposed a vote of thanks to Mr. Kneale for his excellent paper, which MR. GAMBLE seconded. Both gentlemen looked forward to seeing the printed paper, together with graph, formulae, and tables, which could obviously not be read at the meeting.

After the meeting was closed, light refreshments were served.

ERRATA-POWER PUMPS. 81

ERRATA.

: Top of last column of table, for "Absolute Gravity" read "Absolute Viscosity."

Page 25: Fifth line from bottom of page, for "Motor oils and heavy spindle oils" read "Spindle oils."

Page 42: Figs. 2 and 3 are transposed.

Poge 52: In table, column A, Outlet Angle, for "360°" read 4C 360:1

Page 54 : Specific speed formula should read VGPM X RPM

Hs

Library Digitised Collections

Author/s:

Kneale, R. F.

Title:

Transportation through pipe lines of oil and other viscous liquids (Paper & Discussion)

Date:

1940

Persistent Link:

http://hdl.handle.net/11343/24830

File Description:

Transportation through pipe lines of oil and other viscous liquids (Paper & Discussion)

transportation through pipe lines of oil and other viscous ... · on the continent, the...

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