Getting to Grips With... Pump Cavitation
It can be an expensive problem for your business - but what exactly is pump cavitation and how do you avoid it?
1: Get it right at design stage
There is no alternative to getting it right at design stage, regardless of the pressure to reduce costs. If it is necessary to take out two metres of concrete in order to lower the pump suction connection and achieve the correct NPSHa, then my strong advice is to do it or look for an alternative pump location. Good design to avoid cavitation is always the best option.
2: Don’t automatically blame the pump
All too often the pump itself is unfairly blamed for pumping system problems. In actual fact, problems such as cavitation often manifest themselves at the pump but are rarely caused by it. In fact, nine out of ten pump problems are not caused by the pump itself but by issues such poor system design and lack of maintenance.
by Bob Went, Group Consultant, Xylem Water Solutions UK
Pump cavitation occurs when the liquid in a pump turns to a vapour at low pressure. It occurs because there is insufficient pressure at the suction end of the pump, in other words, there is insufficient Net Positive Suction Head available (NPSHa).
When cavitation occurs, air bubbles are created due to low pressure. As the liquid passes from the suction side of the impeller to the delivery side, the bubbles implode. This creates a shockwave that hits the impeller creating pump vibration and mechanical damage, possibly leading to complete failure of the pump at some stage.
Why is it a problem?
Cavitation can have a serious effect on pump operation and lifespan. It can affect many aspects of a pump, but it is often the pump impeller which bears the brunt of its impact.
A relatively new impeller which has suffered from cavitation will often look like it has been in use for many years; the impeller material may be eroded and it can be damaged beyond repair.
Pump problems caused by cavitation, such as vibration, can be severe and may lead to mechanical damage to the pump. Cavitation related problems also have the potential to reduce pump life from circa 10-15 years, down to just two years in extreme cases.
What causes it?
Cavitation occurs in a pump when the temperature and pressure of the liquid at the suction of the impeller equals the vapour pressure. Cavitation can occur at low pressures and normal operating temperatures. Locally it results in the liquid turning to a vapour and creating very high temperatures and pressures, which can reach circa 10,000K and 1GN/m2.
When cavitation occurs bubbles are formed. As the pressure in the pump increases, those bubbles collapse in the form of an implosion, which is equally as violent as an explosion. The implosion causes shockwaves to travel through the liquid and hit the impeller causing mechanical damage.
How do you spot it/check for it?
Cavitation causes pump performance deterioration, mechanical damage, noise and vibration which can ultimately lead to the complete failure of the pump. Often the first sign of a pump problem is a symptom such as vibration. It should be noted that vibration causes problems for many pump components including the shaft, bearings and seals.
How do you avoid it?
Assuming no changes to the suction conditions or liquid properties during operation, cavitation can be avoided most easily during the design stage. The key is to understand Net Positive Suction Head or NPSH and take this into account during design. In order to understand this term more easily it is helpful to break it down into its constituent parts. Net refers to that which is remaining after all deductions have been made, Positive is obvious and Suction Head refers to the pressure at the pump inlet flange.
NPSH is defined as the difference between the pressure available at the pump inlet and the vapour pressure of the liquid. It is important to bear in mind that vapour pressure is different for different liquids and varies with pressure and temperature.
It is also important to remember that the pressure available at the pump inlet is that which remains after account has been taken of all the friction losses, velocity head losses and inlet and outlet losses in the suction pipework of the pumping system.
During design it is therefore necessary to calculate all the friction losses, inlet and outlet losses and process unit losses in the suction pipework and then deduct those losses from the suction head available to the pump. Therefore, at the point where the pump is installed, we are left with a net pressure remaining and available for the pump.
There is no alternative to getting pump system design right at the design stage. During design the value of NPSHa (which is independent of the pump to be selected) can be determined quite easily. Having determined the NPSHa it can then be compared to the NPSHr (the suction head required by the pump) for the types of pumps being considered. If there is insufficient NPSHa it is much easier to make changes to the system at design stage rather than after construction and installation. It is strongly recommended that any changes necessary are made at design stage as any additional costs incurred may pale into insignificance compared to the costs of rectifying an installation with cavitation problems.
How do you fix it?
In the event that pump cavitation is a problem on an existing installation there are essentially only two routes that can be followed to rectify the problem. These are to increase the NPSHa to the pump or decrease NPSHr by the pump.
Options available to increase the NPSHa will depend upon the nature of the system in question. This can include increasing the pressure on the suction end of the pump or reducing the friction losses in the pipework, thereby making more pressure available to the pump. Increasing supply pressure can be achieved by raising the static head of the supply, applying pressure to the supply vessel, using a booster pump, or reducing friction losses in the pipework by using larger diameter pipes or fewer components and fittings. Pressure could be supplied to the supply vessel with the use of a booster pump.
However, these are rarely viable options for an existing installation and nearly always impractical due to space issues, cost and potential disruption. Similarly, it is rarely practical to replace the suction system pipework with a larger diameter. A second option is to replace the existing pumps with pumps that have a lower NPSHr or install parallel pumping using multiple pumps. In many cases the above options may not be viable, and in all cases they may involve considerable cost and disruption.
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