There are valves in pumps, in piston pumps and in diaphragm pumps, essential and hence inherent in the construction of these pumps.

Pumping systems also need valves.

Both these aspects are proposed to be discussed in this article.

It would be good to make a summary list of all such valves.

What is proposed to be discussed is
• significance or purpose of the function
• the types of valves, which serve the function
• comparative consideration of different types of valves

1. Isolation valves

Isolation valves are necessary to be able to isolate the pump or piping section for maintenance work. The valves will be closed, only when the pump or piping section is to be isolated. Otherwise they would be normally open, the “NO” mode
Common types of valves, which are used as isolation valves are gate valves, sluice valves, plug valves, ball valves, butterfly valves.

Every type of valve is made in a number of design variants.

Commonplace plug cocks and taps in households are also isolation valves. They are in Normally Closed “NC” mode.

In pumping systems, isolation valves are used also near to the delivery nozzle of the pump. There they help to start and stop the pump, with minimum load on the motor. In most centrifugal pumps, the load on the motor will be the least, when the delivery valve is shut off. Valves near to the delivery nozzle of the pump help to start and stop the pump with minimum load on the motor. They will be in NO mode, when the pump is running and in NC mode, when the pump is not running.

The logic of the isolation valves near to the delivery nozzle of the pump is appropriate only for centrifugal pumps. But it can prove hazardous with positive displacement pumps.

Most commonly these valves are manually operated. With positive displacement pumps even if the valve was inadvertently not opened in time, the pump will build up very high pressure very fast and can cause blow-off of the pump. Safety relief valves, mentioned at item 5 are essential for positive displacement pumps. Also, the safety relief valves should be nearer to the pump than the isolation valves.

2. Non-return valves

a. Check Valves

Check valves are also used near to the delivery nozzle of the pump. They will be in NO mode when the pump is running and in NC mode when the pump is not running. By that token they serve the same purpose as the isolation valves are near to the delivery nozzle of the pump, ensuring that the pump starts and stops with minimum load on the motor.

That would raise a query, “Is it necessary to use both a check valve and an isolation valve near to the delivery nozzle of the pump, if both valves serve the same purpose?”

The answer is “yes”, because apart from the function of isolation, they have individually some additional functions also.

When a pump is shut down, the liquid in the delivery column would tend to rush back. If one were not to have a non-return valve, the liquid rushing back can cause reverse rotation of the impeller and take the pump into turbine mode and the motor into generator mode. These are unwarranted conditions for the pump, the motor and the power supply. The non-return or check valve checks the back flow. It is necessary that the check should get exercised.

There are many design variants of Check Valves.

RUBBER FLAPPER VALVE
A versatile and cost-effective solution, the Rubber Flapper Check valve provides a smaller stroke than conventional swing check valves, reducing slamming while offering a strong, quick seal.
Valve Features
• 45 degree seat angle reduces slamming
• Virtually maintenance-free
• Drip tight seating

TILTING DISC VALVE
The Tilting Disc can provide an efficient and solid approach to controlling flow reversal, with a low head loss.
Valve Design can have features such as
• 550 seat angle
• 40% size increase through seat
• Field replaceable seat
• Optional Bottom and Top Side Dashpots

SWING CHECK VALVE
Standard AWWA-C-508-1 details characterisitcs desired for arduous check valve applications, including sewage and slurry.

Valve Features
• Large diameter Pivot Shaft construction
• Accepts Air Cushion and Oil Control devices
• Ideal for sewage and slurry

GLOBE SILENT CHECK VALVE
Mounted vertically or horizontally, these valves are designed to close before the pump stops, preventing flow reversal, and eliminating water hammer and system surges.

WAFER CHECK VALVE
Wafer style Check Valves are available in a configuration that allows placement between two flanges.

b. Foot valves

This is listed as a type of non-return valve, because a foot valve is a non-return valve. It checks the priming liquid from draining into the sump. It defines the volume of air to be removed during priming.

The pressure-drop across a foot valve can be a significant component of the frictional losses on the suction side of the pump. In turn a bad foot valve can cause a pump to cavitate. Unduly high friction loss across a foot valve raises the system head on the pump. Consequently the pump gives less discharge at the increased head.

Recognising such unwarranted effect of foot valves, it was thought desirable to have an Indian Standard to define the desired performance of a foot valve. That resulted in formulation of standard IS-10805.

3. Vacuum Priming Valves

With large pumps or for pumps handling hazardous liquids, priming the pump with a foot valve does not become practical and proper. Then vacuum priming is used. It is always tricky to cut off vacuum priming as soon as priming is over. If vacuum priming continues after priming is over, the liquid will tend to ingress into the vacuum source.

Vacuum Priming Valves are used in conjunction with a vacuum source when priming a centrifugal pump. They prevent liquid from getting to a vacuum source. Vacuum Priming Valves are used for both clean water and dirty water applications.

4. ARC valves

Automatic Re-Circulation (ARC) valves are designed to provide protection for centrifugal pumps. It may combine the functions of a check valve, flow sensing device, minimum flow control and pressure letdown into a single valve.

The need for an ARC valve merits some explanation. The basic purpose is to ensure that the flow passing through a pump shall never be less than the Minimum Safe Flow (MSF).

There are many considerations, which govern the MSF for a pump. One consideration is that if the flow passing through a pump is less than MSF, the liquid will experience churning and will get heated, in turn causing the temperature of the pump to also rise.

Rise in temperature of the liquid also raises its vapour pressure. This leads to NPSHa becoming less. If it becomes less than NPSHr, the pump will suffer cavitation.

The radial thrust in the pump is more for all off-design flows. Increase in load on the bearings accompanied by rise in temperature of the pump, affects the bearing life.

6. Pump Protector

A pump protector is an Air Release Valve mounted in parallel with a liquid level sensing device. If the pump should become airbound at the point of application, the level switch will either shut down a pump, or open or close an electric circuit of an alarm system
The purpose of the protector is to save the pump from running dry.

7. Safety Relief Valves

When discussing isolation valves, the need for Safety Relief Valves for positive displacement pumps has been already mentioned. It should be worthwhile to repeat the statement. It was said there, “With positive displacement pumps even if the valve was inadvertently not opened in time, the pump will build up very high pressure very fast and can cause blow-off of the pump. Safety relief valves, mentioned at item 5 are essential for positive displacement pumps. Also, the safety relief valves should be nearer to the pump than the isolation valves.”

8. Flow Control Valves
Globe valves have earned their distinction as the preferred control valve style. The flow path through a globe valve spreads a pressure drop through the entire device, while other valve styles tend to concentrate the pressure drop at the vena contracta. The vena contracta is the place in the final control element where flowing velocity is at its maximum, and pressure is at its minimum. It is the place where phenomena such as "flashing", "choking" and "cavitation" originate. Because it slows pressure drop and recovery rates within its body, the globe valve is more resistant than ball, plug and butterfly valves to those counter-productive, sometimes destructive events. This inherent stabilizing characteristic of the globe valve enhances its ability to control a fluid stream. Additionally, ball valves have inherently higher dB levels of audible noise compared to globe valves, contributing to potential OSHA violations, regardless of the now common characterized trims.
Valve Selection
If a formal valve specification is not available, obtaining most of the following information can get the discussion started.
• Identify if valve is a 2-way valve for flow, pressure or temperature control or a 3-way mixing, bypass or diverting application
• Type of operator – Pneumatic or Electric
• Control Signal (type and value) and any required accessories
• Modulating Service or ON/OFF Service
• Valve size, connections & materials of construction if important
• Flowing Medium (water, steam, air, etc.)
• Flow Rate required (gpm, lb./hr., scfh, etc.)
• Pressure at the valve inlet (psig - Normal & Maximum)
• Pressure drop available fully open (psid)
• Temperature at the valve inlet (deg. F - Normal & Maximum)
One important parameter for selecting a flow control valve is The Flow Coefficient, CV, of the valve. It is a dimensionless value that relates to a valve's flow capacity. Its most basic form is
CV = Q/ DP
where Q=Flow rate and
DP=pressure drop across the valve.

The CV value increases if the flow rate increases or if the DP decreases. A sizing application will have a Required CV while a valve will have a Rated CV.
The valve's rated CV must equal or exceed the required CV. CV can also be expressed one GPM of water at 70F when DP = 1.

10. In-built or inherent valves

It was mentioned in the beginning of this article itself, where it was stated, “There are valves in pumps, in piston pumps and in diaphragm pumps, essential and hence inherent in the construction of these pumps.”

Following explanation of how a diaphragm pump works, will also explain the role of the inlet and outlet valves in the working of a diaphragm pump.

Air is driven into the bottom of the air cylinder, raising the piston inside and lifting the diaphragm. As the diaphragm is raised, the check valve ball on the intake side is lifted and liquid flows into the pump. When the piston has risen to the top, the pump cavity is filled and the pump is ready for discharge.

Compressed air is then forced into the top of the diaphragm chamber, pushing the diaphragm down and evacuating the pump cavity. The check-valve ball on the discharge side is lifted and the pump is ready for the next cycle.

11. Air and Vacuum Valves

Air & Vacuum valves are designed to vent large quantities of air when filling pipelines, and to allow air to re-enter pipelines when draining to prevent vacuum collapse.

Sizing Air & Vacuum Valves
The sizing of an Air and Vacuum Valve is based on the resultant criteria of two operating conditions: filling and draining the pipe line. Each change must be independently considered in order to determine the most appropriate valve size selection. Air will be exhausted from the valve at the same rate at which the pipe line fills with a pressure differential of 2 psi. across the valve.

Meanwhile, the sizing of an Air Release Valve is primarily a judgmental selection based upon experience and a knowledge of the air discharge rates that can be expected under certain field parameters.
Deep Well and Combination Valves follow specific formulas, as well, that are based on pump capacity and exhaust flow.

12. Surge Control Valves

When fluid in motion is abruptly stopped, a hydraulic surge is created in the system. Hydraulic surge is often referred to as "water hammer". The kinetic energy, released as pressure, can spike up to six times the system's operating pressure - destroying system instrumentation, pumps, pipes, fittings, and valves. Without a suppression device, the shock wave travels the length of the pipe back to the pump, then reverses again, oscillating back and forth until friction dissipates the pressure spike or a system component fails.

There are several major culprits that produce this "water hammer" effect - quick closing valves, back surge, pump start up and pump shut down.

Quick closing valves can be defined as valves that close within one and one-half seconds. Quick closing valves have the potential of stopping large volumes of energized fluid, producing violent water hammer. Pump start-up also stops fluid in motion. During start-up, fluid in a pipe is static and must be accelerated: the pumped fluid is abruptly stopped when it contacts the static fluid in the pipe, again creating a shock wave. A surge suppressor will absorb the resistance to acceleration and/or the water hammer surge created in each situation. As the surge enters the suppressor, the gas inside is compressed, the fluid is accumulated and the shock wave is absorbed. When steady system flow rate is achieved, pressure and fluid are slowly released back into the system by the suppressor.

During pump shut-down and in back surge situations fluid is reversed. When a pump is shut-off, fluid will reverse direction due to the differential pressure created by the momentum of the fluid in motion. In fact, if the pressure differential is below the vapor pressure of the fluid, a vapor pocket will form creating even higher transient pressures. The reversed flow creates "water hammer" as it slams into the pump's check valve. This effect is compounded in a back flow situation, where fluid is pumped over an elevation or vertically, creating an increase in power due to the fluid's accelerated velocity.

Surge Check valves are used on the inlet of the Air and Vacuum, Deep Well, Combination, Dual and Universal Valves to help prevent damage when surges occur.