Download - Ergonomics Valve Human Factors Engineering
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