Pilot-Operated Relief Valves

7 Jun

Pilot-operated relief valves are designed to maintain pressure through the use of a small passage to the top of a piston that is connected to the stem such that system pressure closes the main relief valve.  When the small pilot valve opens, pressure is relieved from the piston, and system pressure under the disk opens the main relief valve.  Such pilot valves are typically solenoid- operated, with the energizing signal originating from pressure measuring systems.

Sum m ary

The following important information in this chapter is summarized below.

T ypes of V alves S um m a ry

Gate valves are generally used in systems where low flow resistance for a fully open valve is desired and there is no need to throttle the flow.

Globe valves are used in systems where good throttling characteristics and low seat leakage are desired and a relatively high head loss  in an open valve is acceptable.

Ball valves allow quick, quarter turn on-off operation and have poor throttling characteristics.

Plug valves are often used to direct flow between several different ports through use of a single valve.

Diaphragm valves and pinch valves are used in systems where it is desirable for the entire operating mechanism to be completely isolated from the fluid.

Butterfly valves provide significant advantages over other valve designs in weight, space, and cost for large valve applications.

Check valves automatically open to allow flow in one direction and seat to prevent flow in the reverse direction.

A stop check valve is a combination of a lift check valve and a globe valve and incorporates the characteristics of both.

Safety/relief valves are used to provide automatic overpressurization protection for a system.

V A L VE ACTUATOR S

Some type of actuator is necessary to allow for the positioning of a valve. Actuators vary from simple manual handwheels to relatively complex electrical and hydraulic manipulators.

E O 1.6             D E S C R I B E th e con stru ction an d p rin cip le of op eration for th e follow in g typ es of valve actu ators:

a.          M anual

b .          E lectric  m otor c.      P n eu m atic

d.          H ydrau lic e. Solenoid

Introduction

Valve actuators are selected based upon a number of factors including torque necessary to operate the valve and the need for automatic actuation.  Types of actuators include manual handwheel, manual lever, electrical motor, pneumatic, solenoid, hydraulic piston, and self-actuated. All actuators except manual handwheel and lever are adaptable to automatic actuation.

Manual, Fixed, and Hammer Actuators

Manual actuators are capable of placing the valve in any position but do not permit automatic operation. The most common type mechanical actuator is the handwheel. This type includes handwheels fixed to the stem, hammer handwheels, and handwheels connected to the stem through gears.

Hand wheels Fixed to Stem

As illustrated in Figure 30, handwheels fixed to the stem provide only the mechanical advantage of the wheel. When these valves are exposed to high operating temperatures, valve binding makes operation difficult.

Hammer Hand wheel

As illustrated in Figure 31, the hammer handwheel moves freely through a portion of its turn and then hits against a lug on a secondary wheel. The secondary wheel is attached to the valve stem. With this arrangement, the valve can be pounded shut for tight closure or pounded open if it is stuck shut.

Gears

If additional mechanical advantage is necessary for a manually-operated valve, the valve bonnet is fitted with manually-operated gear heads as illustrated in Figure 32. A special wrench or handwheel attached to the pinion shaft permits one individual to operate the valve when two individuals might be needed without the gear advantage. Because several turns of the pinion are necessary to produce one turn of the valve stem, the operating time of large valves is exceptionally long. The use of portable air motors connected to the pinion shaft decreases the valve operating time.

Electric Motor Actuators

Electric motors permit manual, semi-automatic, and automatic operation of the valve.  Motors are used mostly for open-close functions, although they are adaptable to positioning the valve to any point opening as illustrated in Figure 33.  The motor is usually a, reversible, high speed type connected through a gear train to reduce the motor speed and thereby increase the torque at the stem. Direction of motor rotation determines direction of disk motion. The electrical actuation can be  semi-automatic,  as when  the  motor is started by a control system. A handwheel, which can be engaged to the gear train, provides for manual operating of the valve. Limit switches are normally provided to stop the motor automatically at full open and full closed valve positions. Limit switches  are operated either physically by position of  the valve or torsionally by torque of the motor.

Pneumatic Actuators
Pneumatic actuators as illustrated in Figure 34 provide for automatic or semi- automatic valve operation. These actuators translate an air signal into valve stem motion by air pressure acting on a diaphragm or piston connected to the stem. Pneumatic actuators are used in throttle valves f o r open- clos e positioning where fast action is required. When air pressure closes the valve and spring action opens the valve, the actuator is termed direct- acting. When air pressure opens the valve and spring action closes the valve, the actuator is termed reverse- acting. Duplex actuators have air supplied to both sides of the diaphr agm. T h e differential pressure across the diaphragm positions the valve stem. Automatic operation is provided when the air signals are automatically controlled by circuitry. Semi-automatic operation is provided by manual switches in the circuitry to the air control valves.

Hydraulic Actuators

Hydraulic actuators provide for semi-automatic or automatic positioning of the valve, similar to the pneumatic actuators.   These actuators use a piston to convert a signal pressure into valve stem motion.  Hydraulic fluid is fed to either side of the piston while the other side is drained or bled.   Water or oil is used as the hydraulic fluid.   Solenoid valves are typically used for automatic control of the hydraulic fluid to direct either opening or closing of the valve.  Manual valves  can also  be used  for  controlling the  hydraulic fluid;  thus  providing  semi-automatic operation.

Self-Actuated Valves

Self-actuated valves use the system fluid to position the valve. Relief valves, safety valves, check valves, and steam traps are examples of self-actuated valves. All of these valves use some characteristic of the system fluid to actuate the valve. No source of power outside the system fluid energy is necessary for operation of these valves.

Solenoid Actuated Valves

Solenoid actuated valves provide for automatic open-close valve positioning as illustrated in Figure 35. Most solenoid actuated valves also have a manual override that permits manual positioning of the valve for as long as the override is manually positioned. Solenoids position the valve by attracting a magnetic slug attached to the valve stem. In single solenoid valves, spring pressure acts against the motion of the slug when power is applied to the solenoid. These valves can be arranged such that power to the solenoid either opens or closes the valve. When power to the solenoid is removed, the spring returns the valve to the opposite position. Two solenoids can be used to provide for both opening and closing by applying power to the appropriate solenoid. Single solenoid valves are termed fail open or fail closed depending on the position of the valve with the solenoid de-energized. Fail open solenoid valves are opened by spring pressure and closed by energizing the solenoid. Fail closed solenoid valves are closed by spring pressure and opened by energizing the solenoid. Double solenoid valves typically fail “as is.” That is, the valve position does not change when both solenoids are de-energized. One application of solenoid valves is in air systems such as those used to supply air to pneumatic valve actuators. The solenoid valves are used to control the air supply to the pneumatic actuator and thus the position of the pneumatic actuated valve.

Speed of P ow er A ctuators

Plant safety considerations dictate valve speeds for certain safety-related valves.  Where a system must be very quickly isolated or opened, very fast valve actuation is required.                            Where the opening of a valve results in injection of relatively cold water to a hot system, slower opening is necessary to minimize thermal shock.                                               Engineering design selects the actuator for safety- related valves  based upon speed and power requirements  and availability of  energy to the actuator.

In general, fastest actuation is provided by hydraulic, pneumatic, and solenoid actuators. However, solenoids are not practical for large valves because their size and power requirements would be excessive.   Also, hydraulic and pneumatic actuators require a system for providing hydraulic or pneumatic energy.  The speed of actuation in either case can be set by installing appropriately sized orifices in the hydraulic or pneumatic lines.   In certain cases, the valve is closed by spring pressure, which is opposed by hydraulic or pneumatic pressure to keep the valve open.

Electrical motors provide relatively fast actuation.  Actual valve speed is set by the combination of motor speed and gear ratio.   This combination can be selected to provide full valve travel within a range from about two seconds to several seconds.

V alve P osition Indication

Operators require indication of the position of certain valves to permit knowledgeable operation of the plant.   For such valves, remote valve position indication is provided in the form of position lights that indicate if valves are open or closed. Remote valve position indication circuits use a position detector that senses stem and disk position or actuator position.  One type of  position detector  is  the  mechanical limit  switch, which  is  physically  operated by  valve movement.

Another type is magnetic switches or transformers that sense movement of their magnetic cores, which are physically operated by valve movement.

Local valve position indication refers to some visually discernable characteristic of the valve that indicates valve position.  Rising stem valve position is indicated by the stem position.  Nonrising stem valves sometimes have small mechanical pointers that are operated by the valve actuator simultaneously with valve operation.  Power actuated valves typically have a mechanical pointer that provides local valve position indication.  On the other hand, some valves do not have any feature for position indication.

Summary

The important information in this chapter is summarized below.

V alve A ctuators S um m a ry

Manual actuators are the most common type of valve actuators.  Manual actuators include handwheels attached to the valve stem directly and handwheels attached through gears to provide a mechanical advantage.

Electric motor actuators consist of reversible electric motors connected to the valve stem through a gear train that reduces rotational speed and increases torque.

Pneumatic actuators use air pressure on either one or both sides of a diaphragm to provide the force to position the valve.

Hydraulic actuators use a pressurized liquid on one or both sides of a piston to provide the force required to position the valve.

Solenoid actuators have a magnetic slug attached to the valve stem.  The force to position the valve comes from the magnetic attraction between the slug on the valve stem and the coil of the electromagnet in the valve actuator.

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