Selection of Correct Non Return Valve in a Piping System for Larger Benefits
Mr. Dinesh UpadhyaySales Director- India
CRANE ChemPharma & Energy
Flow Solutions, India

For several decades, Swing Check valves have been largely used in industries to act as Non Return valve in a piping system. However in a sophisticated piping system, it is substantial to select the valves not only functioning as return valve but considering the consequences it can have in the entire piping system safety and in overall cost. End user can advantage significantly by understanding the capabilities, limitations, advantages & disadvantages of the check valve they employ. The article scrutinizes three types of Check valves - Swing Check Valve, Dual Plate Check Valves and Axial Check valves. In order to find the best fit for a given application, several operating parameters need deliberation. All these may not be imperative for a given application but they all play a role in the selection process.

The use of check valves in a piping system is a very common practice, most piping systems depend on fluid moving equipment to energise the media, the protection of such equipment e.g. Pumps and compressor from back flow must be carefully planned in any piping system and Check valves (Non return Valves) are frequently utilised for this purpose. Though these are relatively simply devices from functional standpoint, they are often installed without adequate consideration of their performance characteristics and without proper understanding of the limitation of various designs available. For many decades, Swing Check valves have been predominantly used in industries to act as Non Return valve in a piping system. However in a sophisticated piping system it is important to select the valves not only functioning as return valve but considering the consequences it can have in the entire piping system safety and in overall cost. End user can benefit greatly by understanding the capabilities, limitations, advantages & Disadvantages of the check valve they employ. Often decisions are made on past references and to some extent on marketing. It should however always will be remembered that more often than not, no two piping systems are identical and the Check valve that is adequate for one application may not be suitable for another necessitating valve modification or in some instances a totally different valve design. For simplicity in this article, we will examine three types of Check valves - Swing Check Valve, Dual Plate Check Valves and Axial Check valves. In order to find the best fit for a given application, several operating parameters need consideration. All these may not be important for a given application but they all play a role in the selection process.

The main Criteria's one need to consider while selecting the right type of check valves are:
  • Initial Cost
  • Maintenance Cost
  • Pressuloss and Energy Cost
  • Non Slam Behaviour
  • Flow characteristics
  • Fluid compatibility
Initial Cost: It is the factor of cost user needs to incur in procuring the valve from supplier and cost which he needs to incur in the installation of valve. Now if we compare Swing Check valves V/s Dual Plate check valves weight for 12" size there is difference of almost 30% part of this material saving is consumed in precision machining and lapping to achieve high level of performance over a longer run. Normally sizes 6" and above of Dual plate valve will cost lesser than Swing Check valve. Higher the size and material grade higher is the saving in product. Now in installation also you need lesser supporting structure for Dual Plate due to lesser weight and you need fewer numbers of fasteners to clamp Dual plate with adjacent pipe flanges. So on both counts Dual Plate check scores better over Swing Check. Noz check is a high end valves used for specific application and is costlier to both Swing Check and Dual Plate Check Valve.

Maintenance Cost: It is safe to say that more moving parts in a valve more is the maintenance cost. Swing Check valves if not chosen correctly which is often the case can cost heavily due to immature failure on account of fatigue and slamming. It is important to choose the correct size matching with flow rate, one study points out that almost in 70% cases swing check valve is found oversized causing partial opening resulting into chattering and slamming besides high pressure drop. On other side for Dual plate it is important that valve is chosen from proven manufacturer who has strict control in design and quality as it is spring which is the heart of valve if not chosen correctly may fail prematurely and can cause disruption in operation besides needing a replacement thus adding to the cost of maintenance. Nozzle check valve though has the spring but it works on compression mode unlike torsional mode in Dual plate check valve thus chances of spring failure is minimal. Crane Fluid handling associated with invention of Dual Plate check valves(Mission Valves) has experience in selecting the springs from proven source which are tested for millions and millions of cycle to perform.

Pressure Loss & Energy Cost: Pump or compressors needs to generate the discharge to overcome static and frictional loss of given piping system. While static head is the difference in elevation between the source and highest point of service. The frictional loss is caused by roughness in the pipe and loss by valves and fittings. Valve body geometry dictates the general flow area through the valve and some valves restrict the flow are to below 80% of pipe area thus causing huge head loss, 2nd design of closing member also plays an important role in head loss of valves , if disc swings or rotates out of the flow path can induce lower loss another important factor is special contour and shapes to fully open at low fluid velocity and create a smooth path through the valve thus inducing lower loss.

In correctly engineered Swing Check valve Disc is normally moves out of flow path when in fully open condition but reality is different as in almost 70% cases it is found that it is never 100% open and thus never moves out of flow path fully. Its Disc is heavy and not engineered to open at low fluid velocity. In Dual Plate check valves Plates shape is engineered to achieve smooth path and are also much lighter in weight thus fully opening at low fluid velocity, Highly engineered

Dual Plate Check valves use PTFE/Graphite Sleeve and Long leg Springs to make movement of plates frictionless and highly smooth thus reducing frictional loss to a higher degree

Same is the case with Noz check rather in smaller sizes pressure loss is lower than Dual plate and Swing Check valves. Pressure loss is normally measured by CV flow coefficient which is defined as the amount of water in gallons per minute that will pass through a valve with a 1PSI pressure drop, hence more efficient the valve greater is the CV value. Another coefficient used for calculation of head loss is resistance coefficient Kv which is defined as

The lesson here is while it is important to consider Kv value between type of valves, the head loss between various suppliers of a given valve type does not typically produced significant change in system operation. As head loss is a clear function of fluid velocity so flow condition can affect the valve head loss. Further the velocity may affect the open position of valve. Swing check valve may require between 4-8 Ft/Se of velocity to be forced fully open by the flow. If the valve is not fully open the head loss can be significantly higher than the published head loss, even double. Hence the minimum full open velocity should be used when sizing and computing head loss for swing check valve. The head loss from various valves can be converted into annual energy cost related to electrical power needed by the Pump or compressor to overcome the additional head loss from the valve with the equation

Flow Characteristics; Water hammer and its associated problem of surge pressure, vibration and shock can have serious consequences in a piping system conveying fluid, if check valves are chosen without proper consideration to their dynamic performance. They can be a major contributor to the aforementioned problem. Dynamic performance is a term used to describe how a check valve performs under various pump shutdown situation and normally illustrated by a plot of reverse velocity V/s deceleration (dv/dt). All check valve permit some degree of reverse flow prior to closure in a pump shutdown situation and it is sudden reduction of this backflow to zero which yield a rising surge in pressure, this can be coupled with a pressure drop on the upstream side ,resulting in the formation of vaporise cavities ,compounding the water hammer phenomena and Slamming. The higher the revers velocity, the higher is the surge pressure developed and the higher the impacting force generated upon valve closure. Therefore it is important to minimise the reverse velocity, as the surge pressure generated and impact force may damage the pipe system and excessive noise may be created

Slamming relates more specifically to the valve itself and depends upon the right type of check valve selection. Valve slam occurs after a pump or compressor stops when forward flow decelerates, reverse and accelerate back. The check valves must close quickly before the reverse velocity is too high, in order to minimize the surge pressure and protect the line. Extensive research has been conducted into the dynamic response of all check valves and it has been found that slam can be reduced by improving dynamic response of valve. This is achieved by ensuring that
  • The Disc has low inertia and friction
  • The travel of Disc is short
  • The closure of the disc is assisted with springs
On all above parameters Axial Check(Noz Check) scores high over Swing type and Dual Plate type, another feature this valve has is maintenance free as spring is centrally guided and spring force is on compression unlike in Dual plate check valves where spring force is torsional

As clearly visible Swing check has a poor response to changes in a fluid movement in a pipeline, Spring loaded Wafer Check Valve such as crane Duocheck using a maximum torque independent springs has a better response whereas a system which requires a very fast response Axial (Nozzle)check provide excellent response time, the reason is moving components are of lower mass.have a shorter distance to travel and use of properly size spring is helpful.

In making a decision on what type check valve to use, on basis of dynamic performance, following guidelines can be useful

It should be noted that because the Nozzle check valves has the best dynamic performance, this does not mean it has to be selected for all liquid application. It is more suitable for very rapid deceleration rates specifically in incompressible flow.

Fluid Compatibility: Line media is critical for selection of type of check valve, Normally most type will handle clean media and will need special attention if suspended solid concentration is on higher side, If the valve has a straight smooth flow path, the potential for clogging is greatly reduced, with this mind Dual Plate or nozzle Check valves should not be used for the media containing high solids

Now that the types of check valves and their performances are understood, one needs to apply rational process for specific application that satisfies System parameters. There is no single Check valve that is best for all application. Every Piping systems and Installation will require different weights to be given to selection criteria to arrive best suited type of check valves.