1. Purpose

This document provides guidelines and steps for sizing on/off valve and actuator assemblies.

2. Responsibilities

Instrument & Control (I&C) Engineers are responsible for requesting the relevant process data for each on/off valve from the Process Engineer on the project.  The Process Engineer is responsible for providing in a timely manner accurate process data as requested.

3. Equipment / Software

Depending on project requirements, either Intergraph Smart Plant Instrumentation (SPI) or International Society of Automation (ISA) Microsoft Word data sheets will be used to obtain the process data required to size each valve. 

4. Instructions


Prerequisites to performing on/off valve assembly sizing are a Design Basis, issued P&IDs, defined project units of measurement (°F, PSIG, etc.), piping specifications and instrument tag numbers entered in the instrument data base.  Also, the Service Description and P&ID need to be entered in the instrument database for each tag number.

If using SPI, the SPI administrator will grant the designated process engineer access to the SPI process data module.  The process engineer will input the process data either per line number or per instrument tag number.

If using ISA data sheets, print an ISA data sheet for each tagged on/off valve.  For each on/off valve, put the appropriate tag number, service description and P&ID number on the printed data sheets.


  • Review the printed data sheets and identify the process data required to select each on/off valve.  As a minimum, requested data should include the following type of data for minimum, normal and maximum flow conditions:

Liquid Flow

- Fluid
- Inlet Pressure
- Maximum Inlet Pressure
- Outlet Pressure
- Maximum Shutoff Pressure
- Flow Rate
- Operating Temperature
- Maximum Temperature
- Viscosity

Gas or Vapor Flow

- Fluid
- Inlet Pressure
- Maximum Inlet Pressure
- Outlet Pressure
- Maximum Shutoff Pressure
- Flow Rate
- Operating Temperature
- Maximum Temperature
- Gas Specific Gravity

  • The Process Engineer inputs the requested (as a minimum) data on the data sheets or in the SPI process module.
  • The Process Engineer transmits a list of on/off valves whose process data has been input to the SPI process module or the completed data sheets back to the I&C Engineer.
  • After receiving the process data from the Project Process Engineer, the I&C Engineer then begins the selection process. 
  • Review the process data to see if it is logical and does not contradict other data.  For example, when there are two identical productions trains, similar valves in each train should have the same data.  Or if one device is downstream of another, it should have a lower pressure unless there is an intermediate pump or a compressor.  Discuss any discrepancies with the Process Engineer.
  • Input the process data onto the data sheet.
  • Review vendor’s valve tables and select a valve that is the same size as the pipe where it is installed and meets the piping specifications for pressure and temperature ratings and material.
  • Each valve should be provided with a fail-safe actuator.
  • The actuator should be sized to properly stroke the valve to the fail position with zero downstream pressure and upstream pressure equal to the maximum allowable pressure of the pipe.  Supplied actuator should provide at least 25% more torque than the valve requirement, but should be less than the valve actuator stem torque rating.
  • There are three types of actuators normally provided for on/off valves:

- pneumatic spring return type actuator
- pneumatic double acting actuator
- hydraulic actuator

  • Normally pneumatic, spring return type actuators are preferred.
  • If a pneumatic double acting actuator is selected, it should be provided with a volume bottle of sufficient size to provide two full (open and close) strokes of the actuator.
  • Hydraulic actuators require the installation of a hydraulic power unit (HPU) with accumulators.
  • Pneumatic actuators should be sized for an available air pressure of 80 PSIG, but should be rated for a maximum air pressure of 125 PSIG.
  • Hydraulic actuator sizing depends on the discharge pressure of the HPU.  If no hydraulic pressure data is available, use 1500 PSIG for preliminary sizing. 
  • The I&C Engineer must obtain the torque rating for each valve selected.
  • After obtaining the valve torque ratings, select an actuator from the actuator vendor’s torque tables in accordance with “normally pneumatic, spring return type actuators are preferred”. 
  • Note:  This actuator sizing is preliminary.  The valve/actuator assembly supplier will provide the final, certified actuator sizing.
  • After the valve actuator assembly has been selected, review valve sizes with project team members (process, piping, project) and ensure P&IDs reflect the correct valve data.
  • If project requirements identify specific manufacturers and models, then only use those manufacturers and model numbers, to the extent practical.
  • Package all finalized data sheets together with the selected valve/actuator assembly.  Provide Document Number and Revision per Project requirements.  If there are project specific procedures, include them also.
  • Start document review process per Inter Discipline Check Practice.  If there are project specific procedures, comply with them also.
  • After the document review cycle is complete, resolve any comments, revise and reissue data sheets accordingly.