Ergonomics –

Valve Human Factors

Engineering

Managing the Physical Force

Operators are Required to

Apply

Dale Rhodes, MSPH, CIH

Goals

Reduce/ eliminate worker overexertion

injuries

Reduce/ eliminate worker repetitive motion

injuries

Reduce predictable and controllable risks

preventing reaching 0 recordable injuries.

Knowledge Required

1. What valve turning forces are required?

2. What is the safe strength capacity of the

lowest 95th percentile of the work force?

3. What variables can be controlled to

reduce force requirements to meet #2?

4. What tools and design features need to

be used to control the variables?

Valve Actuators

Automatic actuators:

– used to operate valves automatically and/or remotely.

– Automatic actuators typically use pneumatic, electric

or hydraulic power to actuate a valve shaft.

Manual actuators:

– levers or wheels used to transfer physical force from

the human operator to valve shaft directly or through

mechanical gearing or hydraulics.

Torque and Force

Valve Stem Torque Requirements

Actuator force output requirements – must be sufficient to overcome valve static friction and dynamic

torque.

Static friction: – developed in the metal-to-metal surfaces, seats, and seals.

Dynamic torque: – unbalanced force of the process acting on the plug, disc, or ball.

Valve torque requirements: – supplied by the manufacturer and based on pressure drop

across the valve. A minimum of 10-20% safety factor should be applied to the maximum predicted force requirement to insure reliable operation.

Manufacturer Standards

Manufactures Standardization Society of the

Valve and Fittings Industry, Inc. – “Guidelines for

the Operation of Valves SP-91”

– Typical Operator defined as capable of applying

150# of force based on a 12 inch lever.

– Valve designs standards allow for up to 240# of

required force (18” hand wheel) depending on

wheel size

– Do suppliers even belong to or subscribe to this

minimal protective standard?

Real World – Required Valve Force

Few field studies of required valve turning

forces exist

1992 study; 217 random chemical plant

valves – 93% could be cracked open with

a 3 ft long wrench using 100# or less

force. 7% could not!

Real World - People

1995 study 250 male & female college students, data for optimal valve orientation, 16” wheel – Males could apply 123 – 127.5# peak force by hand

– Females could apply 73 – 100# peak force by hand

1997 study – males could only apply 45 – 55# of sustained force by hand

when turning a valve through 60 deg/sec.

Typical real world required manual valve actuation force is a miss-match with human capabilities.

= Injuries which = not meeting safety metrics goals

Human Operator Force Capability

Variables Affecting Force an Operator can Apply:

whether a wheel, lever or T-lever hand control is used

the shape of that control and effective lever length or wheel diameter.

the horizontal and vertical position of the valve with respect to the operator

the distance from the operators vertical line of balance.

the angle of the valve control with respect to the horizon

Adjustments need to be made in maximum acceptable valve actuation force for each of these factors.

Grip Strength Factors Grip strength magnitude affects the torque that can be transferred to the hand control and valve stem.

For small single hand operated valves, triangular rather than traditional round handle shapes reduce gripping force required by a factor of three for a given amount of applied torque. Square, star, oval and other non-round shapes are also advantageous to application of force.

Max. grip strength for small single hand operated valves with round handles is achieved when the diameter in kept near average palm grip size of 8 to 9 cm (3 to 3.25”) with gloved hand.

For larger rim grip valve wheels, the maximum grip strength is achieved when the rim profile diameter is about 32 mm (1.25”) for gloved hands.

Gripping aids like wide bumps and spokes improve applied wheel force by reducing grip effort.

Work Practices Risk Factors

Reduce force required. Use protocols that decrease physical risk and use body mechanics to greatest advantage. (Safety in Motion).

Always ramp up applied hand/arm force slowly to a valve handle rather than instantly. Force increase = 26%.

Common practice of sudden high force sudden injury

Use “grippy” gloves that increase grip allowing more efficient force transfer (up to 15% increase)

Use wrenches! This decreases force required (torque vs. force formula)

Human Force Capabilities as

Function of Large Valve Location 5th Percentile force in Lbs for 57

random male subjects

Presented as a function of valve

wheel height and angle.

Hand to valve wheel turning

Other data indicates female

strength at 45% of male values.

Valve Location Factor

Valve angle and layout can contribute greatly to ease or difficulty in turning the valve

Leave adequate room for valve wrench use for both large wheel valves and small “hand” valves!!

Small hand valves have different orientation considerations than large valves

“Hand” Valve Positioning

Common hand valve

placement or mode of use

doesn’t match the hand/

wrist safe working range

Operation with

wrist at ≥30o

vertical bend

contributes to

carpal tunnel

and other

hand/arm injury

“Hand” Valve Positioning

“Hand” valves which can only be accessed in

front of the operator must be placed or angled to

allow wrists to have less than a 30o bend.

“Hand” Valve Positioning

If at all possible a small handled “hand” valve should be oriented so the stem forms a near 90o angle with the extended arm.

The ideal small handle can be grasped like a comfortable hand shake.

“Hand” Valve Positioning

Turning a small handled valve

without use of assist tools

always presents some repetitive

motion or strain injury risks.

– Wrist bending during turning motion

– Excessive turning force

Provide valve wrenches

Provide room around valves to

use wrenches

Avoid creating other body

positioning issues

Train operators to reduce risks

Help for Small Hand Valves

• Frequently used, difficult to reach valves increase injury risks

• Move them if possible

• Use extension devices/ tools if they can’t be moved

• Bring the point of actuation out to a safe operator position

Help for Valves

Similar vertical valves

with and without shop-built

extended “T” handles

Help for Valves Force reducing

portable actuators

– Pneumatic

– Hydraulic

Karmsund Maritime Service

E.H. Wachs Co. Easi-Drive

Lakehead

Help for Existing Valves

Lakehead manual ratchet drives

Force reducing

portable actuators

– Mechanical Advantage

Help for Existing Valves

Valve wheel wrenches

Lever handled valves

Assure good footing, non-slip

platforms

Use where space permits

(potential knuckle busters)

Force reducing, simple

leverage