lpg applications- pumps and plants

LPG APPLICATIONS - PUMPS AND PLANTS

STERLING FLUID SYSTEMS GROUP

www.sterlingfluidsystems.com

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LPG Applications Pumps and Plants

CEH liquid gas pumps at the PTT depot, Bangkok

In accordance with Technische Regeln fr Flssiggas TRF 1996-2.1 (Technical regulations for LPG), mainly propane, butane and their mixtures are regarded as liquid gases in this brochure. Other liquefied gases such as ammonia, propylene, methylene etc. can be handled by Sterling SIHI pumps and the following information should also be taken

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During the various stages of distribution, pumps are required to transfer LPG in order to compensate for the pressure losses in the flow process, by the discharge head of the pumps. Under normal atmospheric conditions, liquid gases are gaseous. Depending on the temperature at certain pressures, they can be liquified .

This characteristic feature, together with other physical characteristics of liquid gases, require pumps offering a particular operating performance for this application. It is established that liquid must flow into the pumps, but when installing special pumps in conjunction with a special design of installation (PC plant), static suction is possible.

When operating at positive suction head, the pressure which is feeding the liquid gas to the pump must be higher than the vapour pressure (measurement x in diagram) by a value corresponding to the required suction head. Because of inflow conditions, this overpressure at vapour pressure is necessary to achieve trouble-free operation. Depending on pump design, this overpressure is extremely low in the case of Sterling SIHI pumps.

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At filling stations this overpressure at the vapour pressure is produced in most cases by differences in static head between feed tank and pump. Arrangements such as these create additional installation costs. The required suction head of the pump therefore is a quality factor which is of special significance for pumps handling LPG. These low suction heads are achieved with Sterling SIHI LPG pumps. It is absolutely essential that the required feeding head (NPSHR) of the pump at the suction nozzle is available .

When pumping close to vapour pressure, partial vaporization in the feed pipe, suction pipe, or in the first pump stage occurs frequently, and the pump has to handle a liquid / vapour mixture. This is only possible how-ever if the pump has been designed hydraulically for this application.

The design and pumping principles of Sterling SIHI LPG pumps in the CEH range meet these requirements.

When handling liquid / vapour mixtures, the pumps behave as though they were handling liquid with lower weight corresponding to the specific gravity of the mixture. Characteristic curves for different mixtures are indicated by dotted lines in the above diagram.

Vapour separated in the inlet area is re-condensed in the pump so that vapour-free LPG can be discharged.

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CEH/ 5 pumps for positive suction operation Sterling SIHI pumps of the CEH range have been developed with special consideration being given to the pumping conditions of LPG. They guarantee: 1. Handling entrained vapour at normal duty without interrupting flow. 2. Very low suction heads for trouble-free operation. 3. Characteristic curves which guarantee that the equipment will continue to pump without trouble in the event of increased counter pressure. The pumps are of segmental design, an axial inflow and discharge branch pointing radially upwards. On the suction side, an inducer is fitted making it possible to achieve the favourable suction heads indicated. The side channel stages are arranged in series.

On the discharge side, shaft sealing is by means of a mechanical seal of proven design, which is flushed internally by the pumped liquid and is therefore absolutely maintenance-free. On the suction side, an internal sleeve bearing is provided, this also being lubricated by the pumped liquid. The nominal casing pressure is 40 bar.

The glandless design with isolation shroud has the highest safety requirements. Motor torque is transmitted by a maintenance-free magnetic coupling. The nominal casing pressure is 25 bar (or 40 bar).

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Rating of pumps and aspects of installation Pumps can be selected in accordance with the characteristic curves. Specific data, especially with regard to the required motor, will be given in our quotation. When selecting a pump size, it must always be borne in mind that vapour-free inflow can not always be guaranteed. In practice, it is sometimes advisable to add approximately 20 % to the flow rate and an addition to the discharge head corresponding to 1 or 2 pump stages. The influence of specific gravity on the power absorbed is determined by the following formula:

Aspects on installation (general) 1. The run of the inlet or suction pipes must be such that there is a minimum of resistance. There should be no bends, filters etc. which could hinder flow to the pump. 2. Suitable measures must be taken to maintain the pumping range quoted. 3. Bypass pipes from discharge side to inlet or suction side are not allowed. The bypass backflow from the relief valve must be fed back to the storage tank. 4. If one pump is to fill several tanks with different LPG mixtures at different vapour pressures, efficient non-return valves or other suitable shut-off devices are required on the discharge side to avoid any service liquid blowing out during the change-over process. 5. All relevant regulations are to be observed (e.g. VBG 16, TRB 801).

Aspects on installation for CEH/5 (positive suction head operation) 1. The net positive suction head (NPSH) must be higher than the required NPSH. 2. A 1 to 2 pipe is to be fitted from the suction line close to the suction branch to the vapour phase of the feed tank in order that vapour can escape and not enter the pump. 3. The diameter of the feed pipe is to be equal to the nominal width of the pump.

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The filling valve must have relatively short closing periods to comply with the relevant instructions for cylinder filling. This causes pressure fluctuations in the system, which could be detrimental to the pumps, and may lead to short breaks in operation if a reduction in pressure results in inadmissible evaporation or expansion of the vapour bubbles. This problem is particularly important when filling small bottles. Pressure shocks can be absorbed by installing a bypass from the discharge side of the pump to the storage tank. The return of heated liquid will cause a slight increase in vapour pressure in the feed tank. This will considerably improve suction head conditions. An expansion tank filled with inert gas can be installed in the discharge line to reduce the pressure shocks.

Commissioning The training of personnel working on LPG installations is most important. Mistakes made when starting up can result in dry-running and subsequent problems caused by damaged pump components.

1. The pump must only be started when the pump is correctly vented and filled with liquid. 2. Do not operate against closed discharge valve. Fit an overflow valve. 3. Ensure that the vapour return line from the suction line to the tank always remains open. 4. Do not operate the pump outside its range. 5. Check direction of motor when uncoupled from pump. 6. When draining the tank ensure the pump is switched off (a level monitor or dry-running protection is recommended).

See operating instructions for further information .

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PC units operating on suction lift Space restrictions and technical safety regulations increasingly require the installation of underground tanks or surface-mounted tanks covered with earth. Operating on suction lift required by these con-ditions makes severe demands on the pumps. Sterling SIHI pumps CEH/ 6 and CEH/ 7 in conjunction with PC systems can meet these demands.

Due to their capability of handling large quantities of vapour and their low NPSH, these pumps have the advantage, in that they can be used without having to fit moving components or assemblies inside the pressurised tank. The operating principle of PC systems can be seen by referring to the pressure / temperature diagram. By the exhausting of vapour and re-vaporization during the suction phase heat is drawn from the liquid in the suction pipe.

The pressure difference generated against the constant tank pressure causes the liquid to rise up to the pump and the pumping commences. At the same time, an energy-bearing partial flow is returned to the vapour phase of the tank and continues to produce the differential pressure to overcome the suction lift required. The relatively high consumption of energy caused by partial recirculation sets an economic limit for this system at suction lifts of Hs < 4 m and tank volumes of V < 200 m3 for propane and propane/butane mixtures with at least 20 % of propane.

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Installation for PC-plants on suction lift operation

1. The static suction lift must be limited to the absolute minimum. Hstat. < 4 m.2. On initial start up, the discharge end of the pump should be provided with a facility for venting to atmosphere. 3. In order to guarantee a reliable suction lift operation, a gas separator must be installed at the discharge end. A bypass and a gas separator balancing line must be led from the separator into the vapour phase of the tank. 4. In the retaining stage, the pump has two connections for an explosion proof level switch, making it possible to ensure that the pump can only run when it contains sufficient liquid. 5. A delayed release timer relay set to 30 sec. must be installed. This is to allow for the transient decrease in level within the retaining stage during the suction phase. 6. The SIHI PC plant complies with all the above requirements.

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Vertical tank pumps CEB /ZEB

In storage tanks with diameters not exceeding 6 m tank pumps with externally mounted motors can be used as an alternative. These vertical pumps are mounted inside the storage tank on the dished end and are designed to suit a specific tank. The first pumping stage is located in the bottom of the tank by means of an extension piece and operates under suction head conditions. The following pump types are available dependent on the operating range:

1. Side channel combination pumps using the hydraulics of the CEH pumps up to 35 m3/h

2. Centrifugal pumps using the hydraulics of the ZEA pumps and the low NPSH impeller, up to 100 m3/h

3. Booster pumps with low NPSH impeller for inlet pressure increase.

All pumps are equipped with magnetic couplings and are therefore totally leakfree, safety regulations and environmental considerations are taken into account in the design.

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Installation for CEB / ZEB pumps

When adjusting the min. level switch in the storage tank it is important to take into consideration the required positive suction head.

During standstill, differences in temperature between storage tank and external pump parts may cause the liquid to be forced out of the pump. By installation of balance lines or vapour eliminating by-pass valves a permanent liquid level in the pump can be guaranteed.

If the pump is equipped with a foot valve a safety relief valve is required. (included in the scope of supply).

The pump must be protected against dry running operation and extreme temperature increases by suitable monitoring instruments (e.g. flow indicator, load detector, PT 100 etc.).

Inadmissible modes of operation as exceeding Qmax and Qmin. values must be avoided by suitable measures such as regulation by orifices or a permanent Installation example by-pass. The installation and operating instructions must always be observed.

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Vertical booster pumps CEB/ZEB with combined external pressure generating pumps

Booster pumps are mainly used where it is essential to have as few moving parts as possible inside the storage tank. The single NPSH impeller on this pump is fixed on the shaft and is extremely resistant to wear and operates effectively under unfavourable conditions. The main pressure increase is achieved by means of an externally mounted pump allowing ease of maintenance.This solution has proved successful where different applications have to be combined, e.g. where two pressure generating pumps of different sizes are to be fed by only one feeder pump with dual speed motor to suit different operating conditions.

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Booster pumps with pressure generating pumps

1. Provision must be made in the installation to ensure that both pumps can be started successively.

2. Both pump systems must be protected individually against dry running and increases in temperature.

3. The technical aspects regarding vertical tank pumps and LPG pumps operating under suction head operation also are to be taken into consideration.

4. Documentation for plant design and installation are to be available.

Operation (start up) The training of operating personnel on LPG plants is of considerable importance. Mistakes in start up result in dry running and subsequent operating troubles caused by damaged components in the pump.

The following must therefore be noted: Booster pump CEB with pressure generating pump AEH 1. Start up only when the pump is full of liquid and has been properly vented.

2. Avoid running against closed discharge side, fit an overflow valve.

3. Ensure that the pressure compensating line from the suction end of the pump to the tank remains open at all times.

4. Avoid operating the pump outside its operating range.

5. Check the direction of rotation with the drive disconnected or by using a three phase sequence indicator.

6. Ensure the pump is switched off before the tank is emptied (it is advisable to fit a level switch or dry running protection device).

For other information see the operating instructions.

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Multi stage segmented pumps ZEA /UEA for large flows

For economical pumping where large flows are being handling it is preferable to use multi stage ring section pumps.

In order to meet the requirements of pumping LPG these horizontal pumps have an axial inlet and are fitted with a low NPSH impeller.

The low NPSH required by this series of pumps guarantees operation in many different applications without cavitation and other problems.

The ZEA/UEA-pumps can be fitted with 2 or 4 pole motors and are available with mechanical seals or with seal less magnetic coupling magnetic couplings dependent on the application.

Additionally the UEA range has an integrated side channel stage which is especially suitable for the handling of vapours.

At standstill vaporization in the inflow line may be caused by increased ambient temperatures. The design of the suction stage ensures that vapour bubbles are drawn out of the suction line and vapour blocking is prevented. ZEA-pumps for road tanker filling

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Installation of ZEA/UEA pumps

1. Install a straight section of pipe in the suction line of the same bore as the suction pipe and 10-20 x the diameter.

2. Protect the pump against dry running and extreme increases in temperature (magnetic drive) ~ by using a suitable monitoring instrument (e.g. flow indicator, overload detector, PT 100 etc.).

3. Start the pump only when it is completely full of liquid (ZEA).

4. On UEA pumps it is only necessary to fill the pump up to the shaft centre line.

5. After start up the vapour in the suction line must not be pumped against a higher differential pressure. It is recommended that a temporary by-pass be fitted to the storage tank.

6. The return line from the suction pipe to the suction tank must be kept open at all times.

7. Avoid the operation against a closed discharge by providing a relief valve and return line to the storage tanks.

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LPG PUMPS FOR HIGHER FLOW RATES AND POSITIVE SUCTION OPERATIONLPG PUMPS PROGRAMME ZEA AND PROGRAMME UEA

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Selection

As well as the fundamental properties of LPG pumps, namely

low NPSH

capability to handle vapour mixtures

safety of the shaft sealing

proven material design

the correct selection of the pump size is an essential factor necessary to give an economical system together with good operating reliability.

The determination of the desired output comes mainly from the required discharge times or filling capacities. However, the maximum limits are not necessarily determined by the pump performance but by physical limitations or by the geometry of the other components in the system.

From the capacity curve of a filling carrousel for LPG cylinders the nominal output rating can be seen which with regard to a constant filling pressure should be in-creased by an additional by-pass quantity led back to the feeding tank via a pressure sustaining valve.

The additional quantity is normally from 20 to 50 %, dependent on the size of the filling system. The differential pressure to be generated by the pump depends on the resistance characteristics (system curve), of the LPG plant and of the flow rate.

Whilst the static head may often be neglected, the flow losses (dynamic head) of the components in the system can be calculated accurately with the aid of standard tables and empirical values.

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Uncertainity often arises when calculating the system head which in LPG plants results mainly from pressure differences which are dependent on temperature changes. The seasonal temperature changes and thus the change in the pressure differential must also be considered especially at discharge side because of the increasing number of underground storage tanks being installed when the temperatures are almost constant.The pressure difference can be determined by means of the vapour pressure curve for the media being pumped. The use of the pressure gradient of the liquefied gas has also proved helpful. This indicates the pressure change per C and guarantees a fast and accurate calculation of the differential pressure during the design stage. However it must be realized in principle that if extreme factors are taken into consideration the resultant selection will be an oversized pump. Thus the high cost of both the pump and the operation under the partial load will produce an uneconomic solution and unfavourable operating conditions.

SIHI LPG pumps guarantee reliable filling when moderately rated because of their steep and stable characteristics and also because of their capability of operating for a short time under extreme conditions.

The higher differential pressure necessary to overcome a counter pressure dependent on the temperature is reduced immediately at the beginning of the filling procedure and makes the temporary reduction of the pumping capacity negligible.

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Examples of application industrial plants

The industry uses LPG in different applications e.g.

Municipal gas production

Welding soldering

Double-Duro hardening

Drying ovens

Combustion chambers

Hot-water preparation

Heating of buildings

Automotive fuel

Building construction

Heating of railroad yards

Fuel

Destroying of weeds

Regarding the quantity the consumption of the chemical industry (petrochemistry) takes a special position. For all pumping plants the instructions for installation and start up must be observed.

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Example of application off-loading of rail tankers When off-loading rail tankers trouble may occur if the existing static suction heads are considerably reduced by pressure losses caused by the installation of armatures or extremely long feed lines. Furthermore, it has to be noted that when connecting a number of rail tankers to a common line unequal emptying of the rail tankers will not lead to interruption of the regular flow by vapour locking. (Dry running of the pump)The installation of the pressure balance line between storage tank and rail tanker is essential.

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Example of application automotive refuelling with positive suction head operation The installation of above ground or earth covered tanks with free front side is advantageous for inspection, but requires a large area because of the necessary safety zones. In these cases low suction heads are important in order to keep the installation costs low. The SIHI CEH / 5 pumps comply reliably with these requirements. It is recommended to install a level switch in the feed line which will avoid vapour formation due to a decrease in suction head below the minimum.

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Example for application automotive refuelling with a suction lift operation Lack of space and the requirements for technical safety make underground installation of LPG storage tanks necessary. In this case the pump has to run in suction lift operation. Because of the physical properties of LPG the reliable operation of the refuelling plant using a SIHI CEH / 7 pump is only possible by a special arrangement of the pipework or the use of a SIHI PC plant.

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