Valve Selection Considerations
By TK NG
Valves are generally used for flow or pressure control of fluid. In Fig 1, the amount of fluid flows through the valve can be controlled by varying the plug to seat clearance (i.e. the flow passage) at the valve port. Also, fluid pressure loss across the valve varies with the plug to seat clearance. A valve with suitable port design can therefore be used for flow or pressure control purpose.
When a liquid passes through the valve port, its flow velocity increases due to the more restricted passage and hence the static pressure decreases (Bernoulli’s Principle). Friction loss across the valve and turbulence around the valve port further adds to the pressure reduction. If the liquid velocity, friction and turbulence further increase due to closing of the valve port, albeit liquid flow has been reduced, the static pressure may drop below its own vapour pressure and bubbles begin to form. As liquid moves downstream away from the valve port, its velocity drops due to the larger flow passage and static pressure recovers Note 1. When the liquid pressure rises above the vapour pressure, bubbles collapse or implode and cause shock waves to strike on nearby internal valve surfaces. The liquid velocity and pressure variations through a valve are illustrated in Fig 2.
The above phenomenon is called cavitation. Shock waves caused by implosion that strike on valve internal metal surfaces can result in metal fatigue and damages. Moreover, the valve downstream pressure may fluctuate violently under such condition.
When using a valve for flow or pressure control, the correct type must be selected. We cannot use a gate valve for flow adjustment as it is designed for on-off (open-close) purpose only. Notwithstanding a correct type of valve is used, e.g., globe valve for flow control, an inappropriate valve size may make control over the required flow or pressure range difficult. There are also other considerations such as material, pressure rating, etc. that need to be taken into account when selecting valves.
Considerations for Flow Control
When we talk about selection of flow control valves in a closed loop system such as the chilled water circulation circuit of an air-conditioning system, “valve authority” is a term that we cannot ignore. Valve authority is defined as the ratio of the pressure drop across the fully open control valve (∆Pv) and pressure drop across the rest of the circuit (∆Pc) plus the fully open control valve (∆Pv), i.e. Valve Authority = ∆Pv / (∆Pc + ∆Pv) (see Fig 3).
A control valve of higher authority (0.5 or above) will provide better flow control. It is because closing the valve to produce a higher pressure drop across it will result in a larger percentage increase of pressure drop across the whole circuit as compared to using a valve of lower authority. This is one of the reasons why the size of a control valve may be smaller than the size of the conveying pipe it is connected to. However, a control valve of higher authority means higher overall pressure drop and hence lower energy efficiency.
When a control valve is fully open, it should be capable of handling the highest flow rate of the anticipated range. If the flow has to be adjusted to a lower rate, the valve plug and seat clearance may have to be reduced to an extent that results in cavitation. We must therefore make sure the range of flow rate to be controlled is within the workable range of the valve selected. Otherwise, we may need to change to a smaller valve size or using two valves of smaller size in parallel to handle the large flow range.
Considerations for Pressure Control
For plumbing installation, pressure reducing valves (PRVs) are often used to lower the water supply pressure for normal operation of the connected components and avoiding nuisances such as water hammers etc.
Let us take the direct operated PRV (Fig 4) as an example Note 2. It senses and makes use of the downstream pressure to effect adjustment for achieving the set pressure. The downstream pressure acts directly on the diaphragm and against the spring to move the valve plug for adjusting the flow passage clearance and hence the downstream pressure. Similar to flow control, if a large differential between upstream and downstream pressure is to be effected, the valve plug and seat clearance may have to be reduced to an extent that results in cavitation. In this case, downstream pressure would fluctuate erratically, and valve hunting occurs. Two PRVs connected in series can help get rid of the problem by having smaller upstream and downstream pressure differential across each of them.
There are pilot operated PRVs which are usually adopted for handling larger flow rate and capable of quick response to maintain a closer downstream pressure. Relevant information can easily be found on the Internet.