2 replies to this topic

[arkae]

Dear all,
In a combined cycle power plant, i observed that there are two independent suction lines from the LP drum leading to the pump suction(2 x 100%) pumps. And in the discharge line, the discharge lines of the two pumps form a common header and a single line is taken to the economiser inlets and from there to drums. My doubt here is that why cant we provide a single line from the LP drum then make a common header and provide individual lines to the suction of the two pumps so as to reduce the piping cost. Are there any technical problems?

[kkala]

I have checked a local duty spec for a refinery steam boiler. Separate suction lines for the two boiler feed water (BFW) pumps are not requested. One BFW pump is operating, the other standby. However dedicated minimum flow lines (with yarway type valve) are requested (total two), apparently upstream single BFW line to economizer.
Dedicated suction lines seem too conservative, at least for refinery boilers here, but there may be some reason. For instance:
-A common suction line would have to stop water supply to HRSG, if any of the two valves near and upstream BFW pumps fails (total 3 valves on suction, one near drum). This condition would not stop water supply to HRSG in case of dedicated suction lines (total 4 valves on suction, two near drum). The suction line with failed valve would be isolated, but next suction line / BFW pump could operate.
- NPSHa is critical and in case of common suction line this will be somehow lower (for same drum height), since flow in the common suction line will meet an aditional tee and bend.
- Dedicated suctions offer easier isolation, when one BFW pump has to be serviced (you close two valves).
Opinion of designer should be requested, to know reason of placing two dedicated suction lines. Are also savings due to single suction line important? This can be estimated (suction line is usually short).
This is assumed to concern future works under design (not existing situation).

Edited by kkala, 22 January 2012 - 03:15 PM.

[fallah]

why cant we provide a single line from the LP drum then make a common header and provide individual lines to the suction of the two pumps so as to reduce the piping cost. Are there any technical problems?

arkae,

NPSH provision is critical with hot water as a single boiling point fluid, and cavitation will come on very quickly with such fluids. Hence, the main reason of dedicated suction line for these two parallel pumps may be setting the line pressure loss as low as possible.

Fallah