CONTROL VALVE - SELECTION-SIZING AND SPECIFICATION PART IV

9.1 GENERAL

The accessories discussed in this section should be supplied with the valve. This “total

supply” will allow functional testing at the factory, particularly of stroking times.

The accessories may be installed on a separate mounting plate to allow removal of just the

valve and actuator, thus reducing risk of damage to the accessories.

The requisition shall indicate from which side(s) the valve and its accessories will be

accessible after installation.

Control valve accessories are considered to be instruments and shall therefore comply with

the applicable requirements of DEP 32.31.00.32-Gen.

9.2 VALVE POSITIONERS

Intelligent electro-pneumatic valve positioners (input signal 4 mA to 20 mA) should be used.

If the use of a control valve performance management system is considered, or such a

system is already in use, standardisation on one make of intelligent valve positioner is often

required. In these instances, the control valve specification shall specify details (i.e., make,

model, level of intelligence) of the required intelligent positioner.

If the positioner is not of the same make as the control valve, the control valve

Manufacturer shall be responsible for the correct software settings in the configuration (i.e.

characteristic, stroke etc.) of the positioner.

For split range applications, the range “split” should reside in the DCS and each of the

valves should receive a 4 mA to 20 mA input signal.

The positioner output shall be direct-acting unless otherwise specified.

The positioner shall not be provided with a bypass valve. The positioner shall have a

weatherproof enclosure with a degree of protection of at least IP 55 in accordance with

IEC 60529.

The valve positioner shall have sufficient capacity in both directions for pressuring and

venting the actuator to maintain the specified response times.

The Manufacturer shall, upon request, specify the air quality, consumption and filter

requirements for the elements supplied with the valve.

Tubing between the positioner output and the actuator, or between accessories and the

actuator, shall be as follows:

- the control valve Manufacturer shall determine the tubing diameter so as to achieve the

required stroking times. The minimum tubing diameter shall be 6 mm (1/4 in.). The

tubing shall comply with the specification given in the requisition. Bare copper tubing

shall not be used. For areas with a potential H

offshore application, or other corrosive atmosphere locations, tubing and fittings shall be

of stainless steel or other materials suitable for the stated atmospheric conditions and

shall be subject to the Principal's approval.

- all compression fittings in a plant shall be of the same size, make and type. Mixing of

fittings of different size (e.g. metric with imperial) or make/type will result in unreliable

joints; the make/type is subject to the Principal's approval.

The valve positioner shall be provided with an identification plate, marked with the air

supply pressure and input signal.

The combination of a separate electro-pneumatic convertor and a pneumatic valve

positioner should be avoided. If this is unavoidable, the output range of the converter and

the input required on the pneumatic valve positioner shall be checked in order to prevent

possible instability.

DEP 32.36.01.17-Gen.

April 2003

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9.3 HAND WHEELS

Control valves shall only be provided with a hand wheel if required for operational reasons.

If a hand wheel is required, it shall comply with the requirements specified in the MESC

additional requirements (SPEs).

The hand wheel of a control valve with de-clutch facilities shall be provided with an

instruction plate explaining how it is to be used. When de-clutched, the hand wheel shall

not interfere with the full stroking action of the control valve.

9.4 LIMIT STOPS

Limit stops shall be mechanical devices, mounted on the valve or actuator. They shall be

constructed so that hand wheel operation does not allow travel/rotation beyond the limit

stop setting. Bolts screwed in the body shall not be used as a limit stop.

Limit stops shall be adjustable from outside and fitted with a locking facility, e.g. a locking

nut, to prevent tampering or loosening. The limit stops shall be adequately protected

against unintentional adjustments.

The Manufacturer shall set the limit/travel stops at the required minimum or maximum valve

opening.

9.5 LOCK-UP VALVES

Air lock-up valves shall be specified for all services requiring the control valve to remain in

position in case of instrument air supply failure.

To prevent unintentional adjustments, the lock-up valves shall have a bolt adjustment

provided with a locking facility, e.g. a locking nut.

The lock-up valves shall be set 0.5 bar above the minimum instrument air supply pressure

required by the actuator.

For control valves with a valve positioner, the lock-up valve shall be installed between the

positioner output and the actuator. If lock-up valves are fitted on valves operated by a

solenoid valve, the solenoid valve shall be installed between the lock-up valve and the

actuator.

9.6 LIMIT SWITCHES

If limit switches are required, they shall be of the proximity type. The proximitor(s) and

initiator should be mounted inside a box for mechanical protection. The flying leads of the

proximitor(s) shall terminate in an attached terminal box; alternatively, the protection box

and the terminal box may be combined into one box. These boxes shall be suitable for the

electrical hazardous area classification in which they are located.

If not armoured, the flying leads shall be protected against mechanical damage.

If fitted, the external linkage between the actuator stem or rotary spindle and the initiator

shall be protected against unintentional damage.

Proximity switches shall be in accordance with DIN 19234.

The valve Manufacturer shall follow the mounting instructions supplied by the limit switch

Manufacturer. The valve Manufacturer shall properly adjust the limit switches. The vanes

should approach the proximity switch perpendicular on its axis.

Due consideration shall be given to the discrimination between the “closed” and “not

closed” status of valves, especially for rotary valves without a dead angle, linear motion

valves and valves with quick opening characteristics, with respect to the process

requirements of the flow to be detected.

Proximity switches shall be adjustable and shall function autonomously, e.g., one switch for

the "fully open" position and a separate switch for the "fully closed" position.

DEP 32.36.01.17-Gen.

April 2003

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NOTE: If proximity switches are installed to detect a partially-open valve position, the vane shall be shaped so

that a distinction can be made whether or not the partial open position has been reached/passed.

Example: If an 80 % open position is to be detected, a logic ‘0’ might represent a valve opening

between 0 % and 80 % and a logic ‘1’ not just a valve opening at 80 %, but any valve position of 80 %

and above.

The combination of proximitor and proximitor circuit shall be of fail-safe design for

applications with a Safety Integrity Level 1 and above (see DEP 32.80.10.10-Gen.). For all

other applications, fail-safe design is preferred, but cost aspects may justify the use of

non-fail-safe limit switches.

NOTE: If fail safe and non-fail safe limit switches are applied in a project, the fail-safe design should be used

for all switches connected to the IPS and the non-fail-safe design for all other switches. This practical

approach will provide flexibility during the engineering phase and limits the variety of input cards for

IPS and DCS.

9.7 INSTRUMENT AIR BUFFER VESSELS