Knowledge Required1. 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 ActuatorsAutomatic 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.
Valve Stem Torque RequirementsActuator 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 StandardsManufactures Standardization Society of theValve and Fittings Industry, Inc. – “Guidelines forthe 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 - People1995 study 250 male & female college students, data foroptimal valve orientation, 16” wheel– Males could apply 123 – 127.5# peak force by hand– Females could apply 73 – 100# peak force by hand1997 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 forceis a miss-match with human capabilities.= Injuries which = not meeting safety metrics goals
Human Operator Force CapabilityVariables 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 horizonAdjustments need to be made in maximum acceptable valve actuation force for each of these factors.
Grip Strength FactorsGrip strength magnitude affects the torque that can betransferred to the hand control and valve stem.For small single hand operated valves, triangular ratherthan traditional round handle shapes reduce grippingforce required by a factor of three for a given amount ofapplied torque. Square, star, oval and other non-roundshapes are also advantageous to application of force.Max. grip strength for small single hand operated valveswith round handles is achieved when the diameter inkept 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 gripstrength is achieved when the rim profile diameter isabout 32 mm (1.25”) for gloved hands.Gripping aids like wide bumps and spokes improveapplied wheel force by reducing grip effort.
Work Practices Risk FactorsReduce force required. Use protocols that decreasephysical risk and use body mechanics to greatestadvantage. (Safety in Motion).Always ramp up applied hand/arm force slowly to a valvehandle rather than instantly. Force increase = 26%.Common practice of sudden high force sudden injuryUse “grippy” gloves that increase grip allowing moreefficient force transfer (up to 15% increase)Use wrenches! This decreases force required (torque vs.force formula)
Human Force Capabilities asFunction 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 FactorValve angle and layout can contributegreatly to ease or difficulty in turning thevalveLeave adequate room for valve wrenchuse for both large wheel valves and small“hand” valves!!Small hand valves have differentorientation considerations than largevalves
“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 infront of the operator must be placed or angled toallow wrists to have less than a 30o bend.
“Hand” Valve PositioningIf at all possible a small handled “hand” valveshould be oriented so the stem forms a near 90oangle with the extended arm.The ideal small handle can be grasped like acomfortable 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 – HydraulicKarmsund Maritime Service Lakehead Easi-Drive E.H. Wachs Co.
Help for Existing Valves Force reducing portable actuators – Mechanical Advantage Lakehead manual ratchet drives
Help for Existing Valves Force reducing, simple leverage Valve wheel wrenches Lever handled valves Assure good footing, non-slip platforms Use where space permits (potential knuckle busters)