Actuators must be sized properly for all emergency conditions. Ensure that the sizing and assembly is done only by valve manufacturers and by automation specialists who have demonstrated their capability to the Company.
Ball and plug valve actuator sizing is usually independent of the differential pressure. Butterfly valve sizing is somewhat affected by the differential pressure, while globe and gate valve sizing is greatly affected by differential pressure. Butterfly valves must also be checked for the operating hydrodynamic torque.
Maximum torque for a butterfly valve is normally the opening torque to break the seat. Maximum torque for ball and plug valves is the opening or closing torque. Plug valves that have TFE liners or are fully lined are used in corrosive services. Unless cycled at least once a month, the cold flow properties of teflon may prevent the plug from moving. Maximum thrust for gate valves occurs when closing or seating the wedge.
At a minimum, the actuator must be sized to handle the maximum differential pressure across the valve during an upset condition. To calculate the maximum differential pressure, assume a downstream pressure of zero (simulating a line break or complete depressurizing).
The actuator can be sized for the maximum possible operating pressure, usually according to the setting of a safety relief valve. Whenever possible, anticipate any future operating requirements.
The maximum allowable working pressure (MAWP) of the valve (refer to ANSI B16.34) can be used to size the actuator. The advantage of sizing the actuator with the MAWP is that the valve will work if the process conditions change during plant life. The disadvantage of this method is that the actuator may be much larger than needed for any anticipated operating conditions and therefore more costly to purchase and install. Valve damage may occur at these very high actuator torques. The valve manufacturers should verify that their valves can withstand these hightorque loads.
The vendor should size the actuator, but the engineer must verify the calculations and make sure that the vendor has added the required additional torque or safety factor specified on the data sheets. This safety factor is needed after the valve has been in service. Corrosion or a buildup of deposits make the valve more difficult to seat or unseat.
The safety factors vary depending on the type of valve and service conditions. For piston-actuated, quarter-turn valves, the minimum factor should be 25 percent. For dirty or fouling services, a 50 percent safety factor is often added.
Consider these factors when making the final decision:
• The minimum torque available for various actuator sizes.
• Whether the normal operating or a minimum design pressure will accommodate a dip in the air or gas pressure during an emergency.
• Whether reserve tanks are used.
For motor-actuated valves, a safety factor as high as 100 percent is suggested, because the additional cost of a slightly larger motor is not significant. The horsepower required is directly related to the speed of closing. It is often desirable to decrease the closing time after an EBV is put into service. Unless requested otherwise, vendors typically use closing times of one minute per diameter foot of the valve.
If the MAWP is used for sizing the valves that are affected by differential pressure, then no other safety factors may be needed.