Traditional pump control valves are used for the following reasons:
- Soft start-up of the pump – the valve is initially in a closed position, and then slowly opens. This allows flow into the delivery line to be gradual, without creating water hammer or surge problems.
- Control of Pressure onto the pump – this ensures that the pump does not run off its curve during high demand periods.
In borehole pumps, the pump often has to cope with different levels of head as the water level “draws down”. This means it needs to be protected from initial start-up upthrust damage, as well as keeping the pump on its efficiency point during normal operation. This function has traditionally been performed by pump control valves with pressure sustaining feature.
The problem with this, in borehole pumps, is that when the pressure sustaining setpoint is determined for start-up protection, this same set-point can seriously limit the pump’s output at running conditions. A much better solution is to use a flow control valve, which controls the flow rate rather than the pressure.
A case study (below), presented in some detail in two parts by John Tonkin (one of South Africa’s leading Pump Experts), and with supporting material from Maric Flow Control Australia, shows why a flow control feature is so much more beneficial than a pressure sustaining feature. This case study is summarized below:
The Maric flow control alternative
At pump start-up, the flow is restricted to the pump’s rated flow, preventing up-thrusting and cavitation which can destroy the pump
At duty point, the pump output is less than the rated flow of the Maric flow control valve, resulting in less head-loss than a pressure sustaining valve.
In addition to the obvious benefits of using a flow control feature instead of a pressure sustaining feature, Maric has additional benefits over pilot operated flow control valves: they’re simple, robust and tamper-proof; they have a long life, are not susceptible to dirt, and they require zero maintenance.
Another benefit of using Maric is that the valve can be mounted directly on the discharge of a submersible pump, and it can operate under water. This prevents the valve seeing high air velocities, during start-up, when mounted on the surface.
To gain a full understanding of the underlying principles, please read the articles by John Tonkin of JTA (below) and Grant Schroeder of Maric Flow Control (Australia) (PDF here).
A Video of the Maric’s operation is also available here.
Pump duty point vs duty range by John Tonkin
John Tonkin & Associates
P O Box 5081
Mobile: +27 (0) 71 685 5127
Pump duty point vs duty range: Part 1
There is a common perception in the fluid movement (pump) industry that a selection is done based on “a duty point”. If only life was that simple and change wasn’t an integral part of our daily lives. If fluid systems were approached from a “duty range” perspective, would this be an exercise in semantics or reality? Admittedly some systems do have very small changes in the flow rates and Total Dynamic Heads (TDH) but elements in a system are constantly changing. Tanks/sumps fill and empty, river and dam levels fluctuate and pressures in vessels change with process requirements. The net result of these changes are that operating conditions are created that force a pump to operate far to the left or right of the best efficiency point with serious implications for energy efficiency, Mean Time Between Failures (MTBF) and plant reliability.
Possibly one of the best (or worst!) examples of applications that have large variations in TDH are groundwater or borehole installations. Not only do water levels in the hole vary from season to season but porosity and transmissivity within the aquifer almost always cause significant changes in water levels. Recently we had the opportunity to share in the process of optimising a system that had a static water level of 20m while the level fell a further 65m when the pump was switched on.
The diagram below graphically represents the problem. Curve A shows the system curve for a 32mm class 12 HDPE rising main at a static level of 20m. Curve B is the system curve for the same rising main at the new (dynamic) level of 85m (20m + 65m drawdown). The blue curve shows the pump performance while the dotted red extensions show duty points that are not recommended by the manufacture. At a static head of 20m (start up) the pump will operate at 31l/m against a head of 22m. This is well outside the manufacturer’s recommended range. As the water level falls to the dynamic level (inflow now equals outflow), the duty point becomes more acceptable at 8l/m against a head of 88m. If for any reason the borehole was able to deliver an increased amount of water (higher rainfall, reduction in inflow losses) then the time the pump will spend on the extreme right hand side of the curve will increase. The usual failure symptoms for operation on the right hand side of curve are motor thrust bearing and/or coupling and/or winding failure. The pump end will also have a sharply reduced operating life due to failure of the (up) thrust mechanisms built into the pump end.
While this case study covers a relatively small pump set, it still represents a substantial investment for the end-user. The application could just as well be a multi megawatt shaft dewatering pump. A recent example of this required the water in an abandoned shaft, in the DRC, to be lowered from 120m to 280m. Failure in a system such as this is definitely not an option! Stopping the pump for a matter of hours, allows not only the recovery of the water level but very costly delays in the reopening of the mine. Removal, repair and reinstallation of the pump set and rising main is a hazardous, time consuming and expensive exercise.
Having defined the problem, part two of this posting will look at design options that will prevent pumps and motors from tearing themselves apart.
Pump duty point vs duty range: Part 2
Part one of this posting looked at the problem of selecting pumps that are expected to handle some level of variation in the system Total Dynamic Head (TDH). Having identified the fact that a system will have widely differing TDH values the question arises “what can be done to improve the situation”? In the actual case study covered in Part 1 we had a borehole that had maximum and minimum TDHs that effectively pushed the pump to points that were too far left and right of the Best Efficiency Point (BEP). Some of the usual solutions to this problem are:
- Installing a throttling valve at the discharge. Some of the drawbacks? Unauthorised tampering, use of cheap non control pattern valves (yes this IS a control valve application). If globe, characterised butterfly and ball type valves are used they become expensive and require another level of skill to set and maintain. They are also prone to the “fiddle factor”.
- If the static head is the main culprit (as was the case with our borehole) a smaller rising main can be installed to prevent the pump “falling off” the r.h.s. of the curve. The disadvantage here is that the losses incurred in the pipe remain for both the start up flow and for the minimum flow. Admittedly the losses do decrease as the flow decreases but what is needed here is something with a bit of intelligence!
In searching for an elegant solution to this vexing problem, I came across a valve seems to have some good potential for making a positive contribution to improving overall system efficiency and reliability. The figure below graphically shows the same installation fitted with a Maric control valve.
Curves A & B are the system curves for the 32mm class 12 HDPE rising main. Curves C & D are the curves for a Maric Control valve with a set point of 26l/m. Notice how the curves become very steep as flows approach the rated figure. This is where the work of supporting the pump needs to be done. At lower flows the gradient is flat which is good as this represents lower losses. As the the flow decreases as a result of increasing static heads, so the losses need to be decreased. From the diagram, the head loss at maximum flow is about 22m (the gap between curve A and the intersection of curve C and the blue pump curve). As the water level in the borehole falls, so the Maric valve’s curve rises up. At the minimum flow (8l/m) the gap between pipe curve B and the intersection of Maric curve D and the pump curve is now only +/-3m. Is this the intelligent solution we are looking for with few, if any, of the traditional drawbacks? I would suggest a chat with Peter Telle of Ultra Valves about the specifics of your problem in order to see if this valve can make a positive contribution to reducing cost of ownership in your fluid movement systems.