QUESTION
After checking the ASME PTC4 Power Test Code for Fired Steam Boilers I could not found boiler feed water system balancing procedures.
How should the boiler feed water system be adjusted or balanced to prevent overflow pump conditions?
How to account for delivering proper minimums flows to all boiler feed water valves without overflowing the closer boiler and giving less flow to the farther boiler?
Should a balance valve be throttled before the feed water valve of each boiler to equalize all the boiler pressure drops at full load conditions?
Should this be done with a dummy steam load at start up, or could it be done by pressurizing the boiler before first firing and use cold water only and letting water to flow back to the deaerators?
For example, everybody says to install an orifice or bypass valve at pump discharge to account for dead headed situations. But what about the overflow possibility of a feed water pump?
If for some reason you have three boilers been supplied from two feed water pumps in a common header. The closer boiler will receive more available pressure and flow than the farther boiler when all the feed water valves start opening. Still during the boiler start up almost never will the load be available at the building to simulate the worst case scenario of a 100% load. So the start up is done with the boiler plant at less than full load capacity and the feed water pumps pressure is not equally balanced between boilers.
This procedure should be part of a standard or code to make sure the boiler plants are tested at full load conditions.
REPLIES
Sharik
Either we are doing things far too simply or you are making things far more complicated then they have to be.
At some of our plants we have similar systems, three boilers being fed by two 50% load BFW pumps. The third BFW pump is on standby.
We have never balanced the FW system to ensure there is the same pressure at each boiler FW valve and I don’t see the need to. To set up these plants we take the total steaming load for each boiler, convert that to a total FW flow and ensure we had pumps with performance curves which would ensure, even if all three boiler FW valves were 100% open, the pumps could supply the demand, at the operating pressure.
Each boiler has it’s own FW control valve so there is little chance of overfilling the boiler (well until operations gets too involved!!).
The more critical problem is maintaining a minimum flow through the BFW pumps in the times of low or no demand. Each BFW pump has a minimum flow requirement (set up from the manufacturer) and we have installed automatic flow bypass valves on the BFW pump discharge. This valve opens automatically to maintain the minimum flow through the BFW pump during low demand periods. The discharge from the minimum flow valve goes back to the deaerator storage tank.
So it doesn’t seem to matter if the BFW pressures at each of the boiler FW control valves is the same. As long as the pressure is sufficiently greater than the boiler pressure to ensure enough water can be pumped into the boiler when required. And as long as the boiler feed pumps can maintain this pressure at full flow conditions.
SOURCE
https://www.eng-tips.com/viewthread.cfm?qid=82616
Above is a snippet.