Original pump control were a pneumatic controller for each of the three steam turbine driven pumps in the boiler feedwater system and a master controller. All controllers were identical, and they could be set for either manual or automatic operation they all have Process Variable (PV), Set Point (SP), and Command Output (CO) parameters. The controllers for the two pumps were changed (no PV, SP, or CO on them), and it seems that the control valves for both pumps now operate on whatever pressure is set on the controller(one is 300 psig and the other is 305 psig. The pump left in the original control set up has now two controllers, its controller and the master controller. There is an electric motor driven pump as a back up to steam driven pumps. It is set to 300 psig when it runs by cracking open the discharge valve. The boiler feedwater pressure required is about 297 psig.

Here are questions arising from the new pump control set up.

1. Are there two controllers (including the master) in use for the pump which remained using the original controller? If it is, does manually manipulating the master controller alone (change the Command Output, CO, values) opens or shuts the pump control valve.

2. Are the new controllers for those two pumps still tied to the master controller to have a feedwater pressure of about 297 psig? Likewise, is electric motor driven pump tied to master controller?

3. What is the new control schematic diagram?

 

I would remove all the existing control systems and install the following:

1. One master pressure controller which controls the boiler feedwater pressure.
2. Individual flow controllers (connected to the existing pump flow control valves) on each feedwater pump to regulate the flow.

The master controller will maintain a constant pressure on the header by sending a remote setpoint (RSP) to each controller demanding a flow of water which is the same from all pumps. In this way you can get the pumps to share evenly the feedwater flow demand, irrespective of how many pumps are running. When you want to test a pump, just start it up, and the system will automatically adjust the flows from the other pumps that are running to maintain constant pressure (as long as there are sufficient pumps to cater for the flow required).

The problem with this setup is that you have to have a flow meter installed on the discharge of each pump to be able to set it up.

This setup together with the setup for the pump run commands I recommended on the 6th December to your posting, will ensure that under all circumstances you can have a constant feedwater pressure to your boiler.