Valve wheel rim force capabilities of process operators

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Abstract

The force that an operator is able to apply to a valve wheel depends on the location of the wheel relative to the operator, the orientation of the valve stem, the diameter of the wheel, and the quality of the interface between the wheel and the hands. The objectives of this study were three-fold. The first objective was to provide information on human force capabilities to valve designers to assist them with the design of new valves. The second objective was to assist piping designers with the layout of valves in a new plant. The third objective was to help plant operations staff determine whether the turning force required to operate existing valves exceeds the capability of process operators.

Participants were 66 volunteer process operators and managers from a refinery and a chemical plant located on the Gulf Coast of the United States. Turning strength was measured by recording the force applied to the rim of the valve wheel during an isometric contraction in which there was no movement of the body. The data collected in this study clearly demonstrate a relationship between the posture of the operator, as defined by the height and orientation of the valve wheel, and the maximum turning force that can be applied. For a horizontal stem, the most force could be applied when the valve was in the knee or waist area. For vertical stem valves, operators appear to be able to apply the maximum force when the valve is located between the waist and the shoulder.

Introduction

Workers in process industries are routinely required to manually open and close valves that control the movement of fluids through pipes. The force that is required to manually ‘crack’ and open or close a valve depends on the type of valve and its condition. Jackson, Osburn, Laughery, and Vaubel (1992) reported a study involving participants from a major chemical plant that measured the forces required to ‘crack’ open 217 randomly selected valves. The data indicated that applying 100 pounds (444 N) of force at the end of a 36-inch (90 cm) valve wrench generated sufficient force to crack 93% of the valves chosen for the test.

The force that an operator is able to apply to a valve stem depends on the type of hand control used (e.g. wheel or a handle). For valve wheels, the maximum force that an operator can apply depends on the height of the wheel above the standing surface and the orientation of the valve stem as well as the diameter of the wheel and the quality of the interface between the wheel and the hands. In practice, process operators should not be working near their maximum capability, since the potential for injury increases as operators exert maximum force. So, piping designers should locate valves in positions where the operator can apply the lowest percentage of his or her maximum strength. Alternatively, if the designer must locate a valve at a sub-optimum location, the valve should be chosen to require less force to operate. Today, valves are often located where operators have difficulty reaching them and often require the operators to assume poor postures to operate them. Poor postures combined with high forces are associated with musculoskeletal injuries.

Several studies have investigated the force that can be applied to the rim of a valve wheel in an effort to open or close it. Woldstad, McKulkin, and Bussi (1995) measured the average maximum force (average over a three-second period) and peak isometric forces2 generated by 250 students (125 male and 125 female) on a valve wheel. The task simulated the one-handed operation conducted by railroad workers as they set the brake on a rail car. Results indicated that the turning strengths of males, which ranged from 111 to 115 Nm (82–85 ft lb), were significantly higher than those of females which ranged from 67 to 91 Nm (49–67 ft lb). The data also indicated that the peak force was much higher than the average maximum force suggesting that operators may not be capable of sustaining peak force over a relatively short period of time.

In a study more related to process work, Schulze, Stanton, Patel, and Cheli (1997b) investigated the maximum dynamic torque that could be applied by each of five male subjects to a valve wheel at a nominal turning speed of about 60° per second. Three valve wheel heights, varying between 40.6 cm (16 in.) and 81.28 cm (31 in.) and three wheel angles, varying between 0 (horizontal wheel, vertical stem) and 60° were investigated. The data indicated that the maximum dynamic forces could be obtained at the lower wheel heights, with a horizontal wheel and in the counter-clockwise direction of movement. Maximum torques were in the range of 40–50 Nm (30–37 ft lb). The diameter of the valve wheel was not specified, but from the photos, it appears to be about 40.6 cm (16 in.). No data were provided on the forces that could be applied at each combination of wheel height and stem angle.

In a related study, Schulze, Goldstein, Patel, Stanton, and Woods (1997a) investigated the maximum dynamic torque that could be applied by each of twelve male subjects. Four valve handwheel diameters, varying between 20.3 cm (8 in.) and 40.2 cm (16 in.) were tested along with three handwheel heights — 81 cm (32 in), 102 cm (40 in.) and 122 cm (50 in.) and two stem angles — 0 and 90°. The data showed that the torque that could be applied to the largest handwheel was significantly greater (62 Nm or 45.7 ft lb on average) than that applied to any of the smaller wheels. Torques also increased from 33 to 40 Nm (24–29.5 ft lb) with decreasing wheel height but the differences were not significant. No torque differences were evident between stem angles. Again, no torque data were provided for the combinations of wheel diameter, mounting height or stem angle.

Section snippets

Objective

The objectives of this study were three-fold. The first objective was to provide information on human force capabilities to valve designers to assist them with the design of new valves. For example, the data currently used by the valve industry (MSS, 1992) assumes that operators can apply more force than is reported by the above studies. The second objective was to assist piping designers with the layout of valves in a new plant. The hope is that designers will continue to locate valves where

Participants

Participants were 66 volunteer process operators and managers from two Exxon sites located on the Gulf Coast of the United States. Each participant was given a small gift as a token of appreciation for participating in the study.

Materials

The torque data were measured and stored for later analysis using the ‘PRIMUS’ dynamometer supplied by the Baltimore Therapeutic Equipment Company in Hanover, Maryland. The PRIMUS measures torque with a force transducer attached to a mounting platform and records and

Results

Data were obtained on 66 subjects. Seven subjects were removed because of irregularities in their data. Of the remaining group two females were removed to provide uniform gender data. There were a total of 57 male subjects remaining for analysis.

The data points for analysis were the maximum of the three replicates of the average 3-to-5-second response for each participant at each of the nine valve positions. From this point onward, these values will be referred to as ‘force values’. The data

Conclusions and recommendations

The data collected in this study clearly demonstrate a relationship between the posture of the operator, as defined by the height and orientation of the valve wheel, and the maximum turning force that can be applied. For a vertical stem, the most force could be applied when the valve was in the knee or waist area. The location for maximum force is consistent with the results obtained by Schulze et al. (1997a). For horizontal stem valves, operators appear to be able to apply the maximum force

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Present address: Booz Allen and Hamilton, 8283 Greensboro Dr., McLean, VA, USA.

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