Oliver Brigginshaw opens our focus on pipes, valves, pumps and controls with an explanation of how to cost-effectively operate pumps in a low NPSH environment
Selecting a pump is a balance of many factors, including the volumes and contents to be pumped, the efficiency of the pumps and how frequently the pump will be run.
Where space is at a premium or the cost of changing structures or pipework prohibitive, engineers may also have to deal with a lack of suction static head. Not taking the suction static head fully into account can cause catastrophic cavitation to occur in the pump.
Cavitation occurs when the pump cannot get enough liquid and the resulting reduction in pressure causes liquid to vaporise and form bubbles. These bubbles can grow dramatically and choke an inlet, further reducing the flow of liquid and the performance of the pump.
In addition, these bubbles can implode with tremendous force, literally tearing away at the metal. The resulting increase in vibration and noise can lead to premature component replacement and in some cases complete failure of impellers.
To avoid this catastrophic situation, the pump manufacturer should always ask you for the net positive suction head available at the pump – NPSH(A) – and ensure this exceeds that required by the pump to operate without cavitation occurring – what is known as NPSH(R). See Figure 1, attached.
NPSH(A) is in principle a straightforward calculation, taking into account the suction static head, friction losses, atmospheric pressure and the vapour pressure of the liquid. However, caution must be exercised with the latter parameter, as in an industrial process the liquid may be a cocktail of chemicals and so the vapour pressure may need to be determined experimentally. Also, does the static head change during the process of pumping the liquid, for example during the emptying of a vessel?
For a given NPSH(A), the pump manufacturer will provide a pump with an NPSH(R) less than NPSH(A) by some 0.5M, though if the accuracy of the data is suspect (such as in pumping a cocktail of fluids), then it may be better to increase this difference.
Generally pump manufacturers design their pumps to work at maximum efficiency and hence the lowest running costs. Efficiency is related to pump speed and so the speed is fixed to deliver the maximum efficiency for a pump. These parameters dictate the NPSH(R) for any given pump.
Important as efficiency is in a pump though, it is not the only criterion when selecting the most cost-effective pump for the job. Achieving the necessary static head to run a pump at its optimum efficiency may be either impossible to achieve or could involve costly changes to plant structure, pipework and associated equipment. If faced with this situation, what are the alternatives?
Computer modelling techniques have better enabled us to understand fluid dynamics and how we can design a pump to run efficiently in a low NPSH environment.
However, many of the pumps sold today have designs that date back 50 years and their manufacturers often have do not have the skills to deliver a low NPSH alternative or are not prepared to make the investment to do so.
This means that if you require a low NPSH pump, you need to turn to one of the specialist companies such as Amarinth. Having made significant investments in computer-aided engineering tools and engineering staff skilled in fluid dynamics, these companies are the equivalents of racing car manufacturers.
Using sophisticated tools and techniques they can “tune” existing designs and re-engineer key components to produce pumps capable of delivering the required performance within the NPSH constraints of the plant.
They also have comprehensive test facilities, as finding the point of cavitation reliably is a crucial, but largely empirical, process.
The resulting pump will be one that can deliver the specified performance within the suction static head constraints at an efficiency approaching that of a standard pump.
The cost of undertaking these modifications is of course not cheap, but it is almost always outweighed by the cost or sheer impracticability of modifying the plant structure.
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Oliver Brigginshaw is managing director of pump design and manufacture specialist Amarinth
Original print headline 'How to avoid cavitation'