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Say, there are four boiler feedwater pumps in the feedwater system, three of which are steam turbine driven and one is electric motor driven that acts as a back up for the steam driven pumps. One to two steam driven pumps run depending on the steam demand of the Plant. How would you specify the necessary control to provide uninterrupted supply of 240 psig feedwater to the Plant?

Some questions first:

1. What is the capacity of the electric driven motor feed pump, same as one steam driven pump or larger?
2. Do the steam driven pumps run at constant speed and there is a boiler feedwater control valve?
3. Is the boiler outlet steam flow fairly constant or varying continuously?

I would take this approach:
From the steam flow, and knowing the boiler/steam pump design, I would insert flow switches (on the outlet steam flow) to determine at a point in time how many steam pumps are required in service, e.g. load less than 40% 1 pump, less than 70% two pumps, etc. (I would insert some overlap, i.e. steam flow rising >40% for two pumps, steam flow falling <35% for 1 pump). From the steam pump side you determine how many pumps are actually running (I assume steam pumps have some sort of emergency shut off valve which can be monitored). If the boiler requirement is larger than the pump availability (i.e. boiler requires two pumps but only one pump is running), the electric pump is started. I would recommend that the electric pump is stopped manually after things have been returned to normal state. All this logic can be implemented in a simple PLC.

Such an approach can be tricky to handle if the boiler steam flow varies widely with time, as then the number of pumps required is also varying widely with time. But I hope that this idea can lead you to a solution to your problem.

First off, sorry for my delayed reponse to your questions.

1. We test/switch the feed pumps by putting in service the electric motor driven pump first before we take out the steam driven pump in service. We have three steam driven pumps, one large (in service during summer months) and two small ones. We let the electric motor driven pump run for an hour, then we switch to another steam driven pump. Seems to me that the capacity of the electric motor driven pump is larger than any of the steam driven pumps.

2. There are six steam turbine driven centrifugal water pumps, four large and two small pumps. Two large pumps and a small pump run during winter months and three large pumps and a small pump run during summer months. Yes, the steam driven feed pumps run at a constant speed (3400 rpm for the large feed pump and 3300 rpm for the small feed pumps) and there is a feedwater control valve on each of steam boilers (all six boilers are in service during summer months and four of them during winter months.

3. Boiler steam flow is fairly constant all day. We baseload a boiler when it is necessary.

Further to my comments above, how is the feedwater flow shared between the pumps, as you have multiple pumps running? Does each pump have a seperate flow control valve, or is it simple a case where each pump runs at its nominal speed as set by its governor and the flows simply depend on the internal state of the pump?

It is possible to implement my recommendation on your setup, or are there other constraints?

Wt can be done is u can put a control valve at the discharge of feed water pump whos operation ( i mean opening and closing ) will be controlled by the steam flow, steam temperature and steam temperature. and give a set point to that valve ok it will maintain the flow of water to boiler depending on the requirement ok and further wt u can do is as the flow of the feed water decrease below a certain limit( u can set tht limit depending on the ur boiler requirement) make a logic on PLC so that a command goes to the pumps so that it gets close and again get started a the consumption increases

TC
Vikas