Avoid these common pipeline design mistakes
by Arie Bregman
November 7, 2019

Many times, valve problems are not a function of the valve at all, but stem from problems with the piping system design, valve location, poor installation practices or selecting the wrong valve for the application.

The following information is a guideline of valve basics.

Piping Design: 3 Basic Rules

Rule No. 1: All valves work best when installed in a nonturbulent flow condition.

Strive for smooth, nonturbulent flow in all design cases. Five times the NPS (nominal pipe size—outside diameter [OD] of a pipe) of straight pipe both upstream and down is recommended, but some valve designs can be installed with less straight pipe than that. Acceptability of nonlinear piping is case specific. It is always a good idea to consult the valve manufacturer; they will be able to give guidance to avoid future maintenance problems.

Suggested check valve installation guidelinesImage 1. Suggested check valve installation guidelines. (Images courtesy of DFT Inc.)

Rule No. 2: Use flow calculations to properly size.

Properly sized check valves are essential for optimum system performance with minimum required maintenance. Performing the flow sizing calculations for check valves is as important as doing the sizing calculations for every on/off or control valve in the system. The rules for sizing on/off valves do not apply to check valves, nor do the sizing rules for control valves apply either. Check valves must have a flow coefficient (Cv) less than the Cv calculated when using the desired pressure drop in the flow sizing equations.

It is necessary to determine what minimum flow rates a check valve can handle. Do not assume that the nominal valve size and the nominal pipe size must always match. Always perform any sizing calculations for minimum, maximum and normal flow conditions. Ensure that flow conditions will be sufficient to fully open the valve under minimum and max flow, and normal flow conditions will ensure optimum valve performance with minimum
valve maintenance.

All check valves require pressure from the fluid velocity to open them and maintain a stable position. The minimum pressure to open a check valve is called the cracking pressure, but the differential pressure should be at least two times that to fully open the valve.

Suggested check valve installation when close to pumps, elbows or expansion jointsImage 2. Suggested check valve installation when close to pumps, elbows or expansion joints.

Rule No. 3: Find the proper placement.
A common question is whether horizontal versus vertical line placement matters. A horizontal line is always preferred, because it takes gravity out of the equation. While many styles of check valves will work in horizontal piping, axial flow, or silent, check valves are typical in vertical piping.

Common Pipeline Design Mistakes

Trying to cram valves into small, tight spaces is a mistake that should be avoided. Images 1 and 2 demonstrate a few design rules that follow commonly accepted piping practices.

If pumps, elbows, expansion joints and other valves are upstream of a valve, it can cause turbulence, which can lead to instability of the disc and ultimately lead to wear and component failures.

Installing Swing Check Valves in Vertical Pipe Runs

If the flow in the vertical line is going upwards (against gravity), gravity will cause the disc to slam against the seat when the flow stops, potentially leading to water hammer. If flow is going downwards, a swing check valve will always be open and not doing its job. However, axial flow check valves perform well in vertical piping locations.

In vertical piping, the easiest thing to do is replace the swing check or double door valve with an inline, axial flow-style check valve. The stronger spring of the axial flow valve will ensure that the valve closes before the reverse flow happens. Closing the valve before flow reverses can eliminate most water hammer.

Axial-style check valve springs are sized differently for flow down versus flow up. Flow up can often use a standard cracking pressure spring.

Flow down requires a stronger than standard spring to compensate for the weight of the disc, as well as any static head of the media you may want to keep above the disc.

Low Cracking Pressures

Cracking pressures can be easily accommodated in axial flow check valves, up to a point.

A 3-inch check with a 0.1 pounds per square inch (psi) cracking pressure is possible, but a 30-inch valve with a 0.1 psi cracking pressure is not.

This is a common problem with screwed end valves. Pipe sealant (aka, pipe dope) will migrate into the internal areas of the valve.

This sealant material will cause the sealing mechanisms inside the valve (disc and seat) to not seal and may also cause the valve to stick open or closed.

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