What is bench-set in control valve?

List of contents:

What is Bench-set?

Conditions where bench-set is needed

How to adjust bench-set?

What is Bench-set?

Bench-set is one of the important parameter for control valve. It establish the seating force (seat load) of the plug against the seat when the valve is fully closed.

Proper seating stress is critical for tight shutdown, which in some process facilities has safety consequences.

The bench set is expressed as the range of pressure from the beginning of the actuator to the valve’s nominal travel.

Because the actuator’s spring rates are not very compatible, it is sensible to believe that only one of the bench setpoints can be met and the critical value adjusted should be the one.

This setting is performed on the actuator before it is mounted onto the valve.

Condition where bench-set is needed:

• The actuator diaphragm binds travel at the upper end and the valve plug binds travel at the reduced end if the stem connector is set with the actuator and valve stems spaced too far apart (i.e. the complete length of the stem is too long). The outcome is a valve that can never be completely opened.

• If the stem connector is too closely coupled with the actuator and valve stems (i.e. the total length of the stem is too short), the actuator diaphragm binds travel at the bottom end and the valve plug binds travel at the top end. The outcome is a valve that can never be completely closed.

In such a condition where valve cannot be closed completely, the stem length has been properly set by adjusting the stem connector, the spring adjuster must be set for the proper bench set pressure. This is the pneumatic signal pressure required to lift the plug off the seat. For an air-to-open control valve with a 3 to 15 PSI signal range, the “bench-set” pressure would be 3 PSI

How to adjust bench-set?

To adjust the bench set, Either you remove the valve if the method is not critical and you make a bench adjustment or you make an in-field adjustment in which case you need to consider friction packaging.

Steps for bench set adjustment:

Step 1:

• Provide a means to anchor the actuator, e.g. a vise, and also provide the actuator diaphragm with means of temporary loading pressure. Also provide a licensed pressure gauge (PG) to read the diaphragm pressure from zero correctly through the upper bench set pressure marked on the name plate of the actuator.

Step 2:

Ensure that the actuator stem is at the top of its travel. Set the pressure of the diaphragm to 0psig. Then slowly increase the pressure to the reduced bench set pressure from 0psig while checking for the actuator stem’s first motion. At the reduced bench setting pressure, the actuator stem should indicate motion.

Step 3:

Be sure the spring adjuster is adjusted to meet the requirements of step 2 above

Step 4:

Apply the upper bench set loading pressure to the diaphragm. This pressure expands the stem of the actuator to the valve. Note that the stem of the actuator can slide over the stem of the valve. Use a marker or a cello tape to label the valve stem at the end of the actuator stem.

Step 5:

Slowly reduce the charging pressure of the diaphragm until the lower set pressure of the bench is applied. Measure the distance to the end of the actuator stem between the marker or tape on the valve stem. The range should correspond to the period of travel shown on the travel indicator scale.

Step 6:

If the span is not right, the spring may have been damaged or the incorrect spring may have been mounted on the valve actuator.