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[ayan_dg]

In many cases e.g for Boiler feed pumps it is found that a booster pump is used before the pressure stage pump to increase the NPSHA of the pressure stage pump. My quetion is why the booster pump have lesser NPSHR than the pressure stage pump?

[Art Montemayor]

Booster pumps don’t necessarily have less of a NPSH requirement than the main pressure pump. I don’t know where you have learned that “In many cases ……. it is found that a booster pump is used before the pressure stage pump to increase the NPSHA of the pressure stage pump”.

I believe you will find that all the booster pump does is to make sure that any pressure losses in suction line of the main, pressure stage pump do not cause a loss in NPSHa such that this value is reduced to a level that is less than the NPSH required by the same main, pressure stage pump. The suction condtions of the booster are not necessarily the same as those of the pressure stage pump. And even if they were, there are ways that a pump can be fabricated such that it requires less NPSH (in the case of the booster). However, as is the case in almost all engineering options, there is a trade-off to be paid and where there is a need to have a booster with a lower NPSHr a price must be paid. One means to achieve this is with the pump’s specific speed. Other means are the mechanical design of the impeller and the casing.

Suffice it to say that there are ways – in some applications – to reduce the NPSHr, but one has to have total scope details and process information to evaluate if the application is possible. In most cases, the booster simply makes up for line losses to the main pressure pump and to ensure that these do not increase to the point where the main pump fails due to suction NPSHa.

There are many applications where centrifugal pumps are used in series – not for the purpose of ensuring a good NPSHa, but for increasing the downstream pressure beyond the capabilities of either pump under different conditions. And this certainly is a possibility of a boiler feed water application.