Pressure Regulating and Back Pressure Valves

Comparison of valves with application examples

Pressure regulating valves and back pressure valves look similar and sometimes sit on the same skid. This article describes both and gives examples so you can pick the right one for the job.

Pressure Regulating Valves

They are a common accessory on positive-displacement pumps because discharge pressure can climb high enough to damage hose, valves, filter housings and almost any fitting or instrument downstream.

A regulating valve holds downstream pressure inside the range the process can stand. It opens in proportion to pressure and bypasses enough flow so the maximum is not exceeded.

Constant Pressure or Maximum Pressure?

A positive-displacement pump puts out a fixed volume per shaft revolution. Where pressure itself is not the point — transfer and many metering jobs — the valve is there only to stop over-pressure. Where the process needs a set pressure while flow changes, the same valve holds that pressure.

Constant-pressure jobs across a range of flows include CNC machines with several tool sizes, high-pressure wash with more than one wand or station, and evaporative-cooling nozzles that have to keep droplet size when more atomizing nozzles come on line.

Safety and Relief Valves

“Regulating,” “relief” and “safety” get used as if they were the same part. Decide how you need the valve to act on this application.

Safety and relief valves often dump the line at a set threshold. Some designs do not reseat by themselves. They are used with a regulating valve as a second line of defense if the regulator fails.

A regulating valve sized for the pump should pass full pump flow through the bypass, but it will not drop line pressure below the set point.

Flow through the valve body is open. When line pressure compresses the spring and lifts the plunger, part of the flow leaves the bypass. That line usually goes back to the feed tank. Run it below the low liquid level and as far from the tank outlet as you can, so you do not pull air or swirl into the pump suction. If you tee the bypass into the inlet pipe, keep that tee at least 10 pipe diameters from the pump inlet.

Back Pressure Valves

A back pressure valve puts more restriction on the discharge than the process would by itself. That drop sits at the valve outlet.

Unlike a regulating valve, the path through a back pressure valve starts closed. Pressure has to compress the spring and lift the plunger before anything passes. It behaves like a check valve except the spring is adjustable and it does not set flow direction.

Back pressure valves are for processes that need a minimum pressure. On a Hydra-Cell Pro pump they also improve volumetric efficiency when discharge is held at 30 PSI or more.

Pumps that use spring-loaded check valves seat those valves many times per second. A Hydra-Cell Pro at 1,800 RPM seats 30 times a second. The check-valve spring helps; discharge pressure above 30 PSI helps more. Run the same pump under 30 PSI with no back pressure valve and flow accuracy can move outside the ±1% in API 675.

Even when the liquid does not need high pressure to move, you may still want both valves. Metering food coloring into a mix tank is a typical case: the volume has to be exact, but the pump only has to beat atmospheric pressure. The back pressure valve gives the check valves the 30 PSI they want. The regulating valve stops a spike if someone shuts a valve downstream. Positive-displacement pumps can spike almost instantly when flow is blocked.

The components in that sketch — Y-strainers, ball valves, metering pumps, PLC pump controls and gauges — are covered on fdpp.com and on this site.

Related Articles

• Difference between pressure regulating and pressure relief valves
• Comparison of C20 and C60 series pressure regulating valves
• Sealless pressure regulating valves overview
• Hydra-Cell Pro metering pumps