Valve Flow Arrows | Blog No. 137

Direction of flow is one of the factors that go into a valve’s design.  Valves have a direction-of-flow arrow either as part of the valve’s forging or markings.  We use these arrows to ensure the valve is installed properly.  But does that mean that the valve will only operate properly if installed with the arrow matching the flow of the pipe contents?  Can valves be installed backwards?

Valves will work when installed in either direction.  They are normally installed with the arrow and flow direction going the same way. But sometimes, it is preferable to install with the arrow reversed for operational purposes.  If pressure is on the “downstream” side of the valve, then installing it reversed will prevent pressure on the backseat of the valve.  In some cases, reverse installation will place the valve handle in a position that will allow better access.

So how do we judge whether a valve is installed properly if the arrow is reverse that of the flow direction?  IIAR-2 provides guidance on that very topic.

ANSI/IIAR 2-2025             

13.3.2.1 *Where a valve is deliberately specified for use with the directional indicator marked by the manufacturer being opposite of the normal direction of flow, or the intended vertical, horizontal or rotational orientation is installed differently from the manufacturer's specification, the system design shall specify the intended installation direction and orientation provided that the alternative specification maintains effective and safe operation of the valves and the system.

A.13.3.2.1 This section provides for situations where a designer chooses to install a directional valve in a backwards orientation, which is a method that is sometimes used to provide a high level of resistance to backflow.

The paragraph states “the system design shall specify the intended installation direction…”.  A note should be included on the P&ID that the valve is to be installed for reverse flow.  It should also be noted in the Operating Procedure that includes that valve.

Another reason to install backwards is to have the valve handle in a position that is easy to operate.  It may require installing the valve in reverse flow to achieve this orientation.  90° Angle valves have flow arrows also. It is not uncommon for the valve to be installed backwards so the handle is in an easier-to-operate orientation.  The P&ID should note that the valve is installed backwards intentionally.

If the arrow is opposite the flow direction of the pipe, then ask why.  It should be noted that inspections during fabrication would have caught an inadvertent backwards installation. If it was the intent of the designer that the valve’s arrow be backwards, then the design drawings should be explicit in calling this out.

Would I call this out as a finding on a mechanical integrity inspection?  Probably not, but I would make a note and discuss it with the facility.  It will likely not do any harm, but the facility can analyze and take action if needed.

Previous Blog: Pressure Relief Valves in Ammonia Systems: The Last Line of Defense | Blog No. 136

---

For comprehensive training on Anhydrous Ammonia, click here for our PSM Academy Ammonia Awareness training to learn and earn a certificate of completion. Training is in English and Spanish. Use code SDS20 for a 20% discount on the entire purchase. For more information, email us at academy@machapsm.com.For a comprehensive training on Anhydrous Ammonia, click here for our PSM Academy Ammonia Awareness training to learn and earn a certificate of completion. Training is in English and Spanish. Use code SDS20 for a 20% discount on the entire purchase. For more information, email us at academy@machapsm.com.