Level Control Systems for Reservoirs
Choose the wrong level control valve and it could fail in weeks.
There are many ways of controlling reservoir levels but the basic hydraulic laws apply to all of them. Pressure and flow rate have to be considered to avoid a disaster.
Lets start with by taking a look at pressure.
In a reservoir application anything above 4-5bar is considered high and 6-8bar requires extra precautions.
Pressure changes can cause cavitation. And cavitation can erode valve bodies in a matter of weeks.
As most Engineers will tell anyone who gives them a chance, cavitation is the phenomena of bubbles forming at low pressures (below the vapour pressure of water and which is below atmospheric pressure at ambient temperatures) and “collapsing” again when the pressure builds up.
This collapsing of the bubbles causes a high velocity jet within the bubble. If that happens anywhere close to the walls (or any metallic part) of a valve it causes severe erosion.
The simplest way to determine a valve’s ability to resist cavitation is to look at the ratio between upstream and downstream pressure. Self-actuated globe control valves are fine at an upstream to downstream pressure ratio of 3:1. Some diaphragm actuated globe control valves are even better at 4:1.
This means that if you have an upstream pressure of 12bar you need a downstream pressure of 3-4bar for the valve to be safe. This is a problem in many reservoirs where the downstream pressure is low.
Consider a 10m high reservoir. That’s only 1bar of back pressure. Conventional valves (3:1 or 4:1 ratio) would only be safe from cavitation with an upstream pressure of 3-4bar. Not nearly high enough for most applications.
The C-valve can handle a much higher upstream to downstream pressure ratio – 12:1. This would be a better choice as a reservoir control valve allowing for a dynamic upstream pressure of up to 12 bar. It does this without any special trim. It has an axial flow pattern which results in better flow characteristics than a standard globe valve. This valve has recently become locally available in a self-actuated version.
The next thing to consider is flow rate.
High flow rates (or velocities) cause problems. The valve specification will usually give a maximum acceptable velocity. This ensures good controllability, long life and low noise levels.
For standard diaphragm-actuated globe pattern valves this is around 6 m/s for 24/7 operation. Going up by 20% for a few hours a day is usually acceptable. Axial flow type control valves can accept velocities of 8 m/s because the flow pattern through the valve is better.
Lets take a look at a typical example.
If one applies the sizing formulae for control valves ie Q = Cv √ dP
Q = Flow rate
Cv = the wide open capacity for any particular make and size of valve
dP = the differential pressure across the valve.
The flow rate will increase as differential pressure increases if the valve is allowed to go into the wide open position.
Do the calculation a few times and you’ll see that the maximum recommended flow rate is reached when the differential pressure is around 1 bar. This means that if a valve is allowed to go wide open with a differential pressure of more than 1 bar the flow rate will be higher than the recommended maximum.
Lets look at our 10m reservoir example again. Take a 200mm diaphragm-actuated globe control valve. It can handle a 4:1 pressure drop ratio. If it was allowed to go wide open with 4bar upstream and 1bar back pressure the flow rate would be 329l/s. This is much higher than the recommended flow rate of 200l/s. Even though this valve is operating within the safe pressure drop range it’s way outside the recommended flow rate. The result: early failure and a red-faced Engineer.
The solution is to make sure that the valve operates within the recommended pressure drop ratio and flow rate. There are a couple of ways to do this.
- Flow Limiter. If the dynamic head is more than 2bar and the static head of the reservoir is less than 10m we recommended a “Rate of Flow” control feature on the valve. This keeps the flow rate below the danger level.
- Pressure drop ratio limiter. This gets a little more complicated and one needs to use good practice “rule of thumb” laws.
- Back Pressure device. An orifice plate can be used to increase the back pressure. But, you need to fix the flow rate first to be able to size it correctly. For example, our valve can handle a 3:1 pressure drop ratio. If the dynamic upstream pressure is 9bar and the head from the reservoir is only 1bar we’d need to create an extra 2bar back pressure to get to that 3:1 ratio. Once you’ve fixed the flow rate you’d be able to size an orifice plate to produce the required pressure drop at that flow rate. This solution needs extra thought where the valve has to open and close slowly to prevent waterhammer. The orrifice plate doesn’t produce enough back pressure during the opening and closing cycles. Here one has to use some judgement to decide whether the excess pressure drop and the time the valve has to endure this is sufficient to cause enough damage to reduce valve life significantly enough to be unacceptable. In that case another solution has to be investigated.
- Pressure Reducing valve. The ultimate solution (although expensive) is to install a Pressure Reducing valve upstream of the Level Control valve with a Rate of Flow Control feature on it. This solution will ensure that the valves are operating within their capabilities throughout the range and during opening and closing cycles. In the case of valves that can handle a 3:1 ratio this means that if the reservoir back pressure is 1bar, a dynamic upstream pressure of 9bar can be handled without the valve’s lifespan being compromised.
Although Water Control valves are designed to destroy energy and in some cases handle pressures of up to 250bar (Mining Industry and Power Stations), these control valves become very vulnerable to cavitation and velocity damage on low pressure applications such as Level Control. It is the Engineer’s duty to design around these problems to ensure that the Level Control valve does not operate outside its capability parameters.