Boiler Feedwater Pump

If you place more of a demand on your boiler than condenser/feedwater can keep up with, it will trip off. (Good Thing).

Run your system within its intended parameters and you should be fine.

How is the level for the feed water pump to run sensed? If pumps is not running when the boiler shuts down on low level. Level sensor maybe faulty or the starter for the pump.

If the pump is running the you are either drawing too much steam from the boiler for the pump to keep up or the pump is worn out and can't keep up. Have you increased steam demand by adding additional equipment?

A couple thoughts come to mind...

If the boiler pressure dropped significantly and the level control valve was wide open (due to the low level), the pump may have been operating without adequate NPSH (the higher the flow the greater the NPSHr) even though the DA was at normal operating pressure/temperature.

You said "no feed flow"... is it simply possible the high steam demand exceeded the capacity of the feed pump?

Does your feed pump have a recirc line?

It is standard and required by code (here in the US) to have a locked open recirc line (from the discharge of the feed pump back to the DA). The evolution of this was finding that during periods of no feed demand that a dead headed feed pump would quickly vapor bind and lead to a loss of feed.

In my experience, troubleshooting an issue is best done by taking the childhood behavior of asking "WHY?" relentlessly... The boiler tripped. WHY? It had a Low-level. WHY? The feed pump breaker tripped. WHY? The pump seized. WHY? Bearing failure. Why did the bearing fail? The bearing lacked oil.... etc, etc, etc. until you find the true root cause.

The second most important question is always "What has been done recently?" Valve line-ups, maintenance, operations, etc... anything, even routine things, often point to the problem. This often leads to the answer to one of the above "whys".

Best of luck and let us know what you find!

If the system was running well prior to this incident one would assume that the initial design was acceptable, therefore, one should look into any recent changes or occurrences. The key words I see in your statement are "almost normal". Most BFW pumps do not have a lot of NPSHA. If your temperature is just a little higher, your pressure a little lower and your flow increased due to "high steam demand" then you could have easily experienced cavitation and become vapor bound and lost flow so that your boiler would shut down on low level. This is safer then burning out some tubes. The problem may not be in your piping except at high flow requirements.

Is it possible to run your other BFW pump at the same time? Running in parallel with each at half capacity may keep the NPSHR well below the NPSHA with the individual suction piping not creating excessive losses. This could be done only when a high steam demand is anticipated.

What is the maximum flowrate (MCR) that the boiler was designed for and where were you running at the time of the trip ? What was the boiler pressure at the time of trip ?

Do you have boiler feedpump curves (or were they discarded by the former plant owners) Where,, on the curve, is the pump supposed to be operating at MCR ?

Is there problem with the feedwater contol valve ? Is the valve operating out of desgn range ? What is the maximum capacity and design pressure of the boiler ?

Is your boiler 40, 50 or 60 years old ?

Do you have pressure indication downstream of the pump, or is that missing ?

Are there old PIDs available to enable you to understand how the system is to work ?

Do you have safety valves on the boiler and have they been recently checked for setpoint ?

Do you have a deaerator and storage tank? Is there level indication ?

If the boiler operating pressure is still the same and the pump is used still the same, and the dearator pressure and temperature are still the same so the only thing that could make the boiler feed water is not fed to your boiler is the boiler feed water pressure is not good enough to against the boiler pressure due to pump problem

I do not see the need to modificate the piping system, the only thing root couse of the problem is your pump characteristic is cahnge due to impeller got erosion corrosion so that it make change the NPSH of the pump and make it cavitate.

My suggestion please check the pump impeller if your pump is centrifugal pump, but if it is positive displacement pump please check the stroke.

Hopefully success.

First of all you did not let us know your Boiler size, capacity, your feed pump KW, type and design. Each boiler sizes have different pumps. So we need to know all the information before we can give our suggestion to you.

These are details we should consider.Boiler capacity:66T/Hr.Pressure : 43 Barg.Fw pump kw:250,type:centrifugal:Pump running pressure : 50 barg,Pump capacity:80m3/hr.Feedwater Cv of Open/Close type.Recirculation piping:3/4 inch dia from discharge to suction side not back to deaerator.Steam demand by Paper Machine is up to 20kg/s before back to normal at 15kg/s to 17kg/s.We have 2 feedwater pumps,one running one standby,inverter controlled.

At times motor inverter indicates above 90% meaning almost full speed but flow indication showing 0% flow.(I noticed during one occasion but luckily boiler did not trip as the same pump managed to supply feedwater and maintained drum level).

Steam flow is very critical to paper Machine as the management set 0% steam interruption?.

Hope the others can view my feedback

Again... think of things that have been done in the past few weeks. If the situation you originally described is standard operation and has been standard operation for the past year, then its likely something happened in the past couple days/weeks such as maintenance or valve line ups, etc. It could be something as simple as someone "tweaking" the plant. You mentioned in the OP that the DA were parameters were "almost normal"... a slightly increased temp and a slightly lower pressure could feasibly lead to not enough NPSH for the feed pump and subsequent cavitation and loss of feed. Review ANYTHING that has been done recently... start with corrective maintenance, preventative maintenance, non-routine operations, routine operations. I'll almost guarantee you'll find the answer.

I saw this too PW. My experience kind of over-rode a comment on it though. Generally, the feed pumps can handle design boiler flow (plus additionals, blow downs, leakage, etc) and a healthy buffer (10 to 30%). This case is about the same. While we don't have the pump curves and operating pressures to confirm, the feed pump capacity is given at "80T/hr" or ~20kg/s (oddly equivalent to the peak steam demand, which tells me a bean counter somewhere is pushing things). The boiler itself would simply operate at a lower pressure and likely produce a lower quality steam.

I do give you credit though, once the details are ironed out it might just be a case of the tide going out...

GA and Great explanation PWS.... mortgaging ... tides... self-disassembly. Thank you for sharing your insights.

The 2 things that concern me here are 1) registering 0% flow on your BFW and 2) recirculating some discharge through a 3/4 inch pipe back to suction rather than into your de-aerator drum.

When you put energy into the hot water to increase the pressure you increase the enthalpy and now you put it back to the pump suction probably through an orifice to drop the pressure. Sounds like a very interesting combination of events. I'm surprised the pump is still on the foundation. Do you need a stethoscope to hear the cavitation or is it evident without one? Can you feel vibration at the suction end bearings during the 0% flow period?

I would repipe the recirculation back to the de-aerator as one recommendation rather than to the suction area. A vapor bound pump would probably register 0% flow.

Base on the details above, your boiler design system should not be a problem. Since you have 2 feed water pumps where you have 1 in running and the other is stand by and I guess both feed water pumps are connected in parallel.

You also mention that "At times motor inverter indicates above 90% meaning almost full speed but flow indication showing 0% flow" This part is very interesting.

When you have 2 feed water pump for your boilers, both pumps are connected in parallel. Therefore, each feed water pump outlet must have a check valves to prevent the other pump pumping the water back to the other pumps. So the 1^st things you have to do is to checked your feed water pump check valves. Make sure the both check valves are working properly.

The 2^nd checking you must do is to check your water feed pump pipe that connected to your boiler. There is also another check valves at the inlet pipe to your boiler. That check valves is to prevent the steam flowing back in to your pipe line. If the check valves is not in a good condition, the steam from your boilers will flow back to your water pipe line and caused the air lock inside your pumps. This problem will also cause your pump running but there is no water flow.

There is another check to check your water strainer at your inlet water pipe. The strainer may be dirty and chock up.

Try to do these checking first and see if there is any problem with these check valve.

The other thing that you have to do is to add another pressure switch at the outlet of your water pump pipe line and see your feed water pump pressure. If the pressure drop and your feed water pump is not at 50 bar that means that your pump is not working at it rated capacity. This means that your pump need to be service and some part of the pumps need to be replace. It may be due to worn and tear.

Try out these checking first.

You have already said it. Its the high steam demand which translates to your pump not coping. Two options I can suggest are to either install another pump to assist the first one in times of high demand or upgrade the old one to a higher rating for it to cope during high steam demand take you pick

It sounds like your system was marginal from the start. That is, your average demand is nearly equal to your average delivery. The problem is instantaneous demand.

Check your TDS at the load. If it is over 100 micromhos you are likely pulling water from the boiler with the instantaneous surge. You would benefit from a wet accumulator between the boiler and load. If properly sized it will be half full of water at the flashpoint of your system. The instant there is load the downstream pressure will drop and the water will flash to steam. You will have no steam interruptions and your management will be happy!

More questions...

Was the plant steady state for days/weeks or did this occur during a start-up?

Do you have low feedwater pressure alarm to alert operators that a loss of feed has occurred?

Is there a feature for the stand-by feed pump auto start? If so, did it?

What is the suction temperature of the feed water pump?

If you have condenser and use the condensed steam, your feed-water suction temperature would normally high. When the steam demand is high and speed of the pump is also high as to maintain the level, the throughput of the pump would also tends to be high. This leads to very low suction pressure, since the suction supply/feed system remains the same. At this low pressure and high suction temperature, the water is likely to boil and bust to vapor. This condition is cavitation and once it cavitate heavily, only vapor would occupies by and large in side the pump and water would not flow effectively. In such a situation the following aspects can be looked into:

1. Try reducing the suction water temperature.
2. Try to improve available NPSH by sizing up the suction line.

Good luck.

It is well known that adding cold water into boiler causes drop of water level, because prevents forming bubbles of steam, while prior to the feed the steam bubbling lift the water level surface up, so the boiler "thought" it was proper level.

Feeding-water cools the heating surface of boiler and that stops bubbling and water surface drops down abruptly, so the tripping. Introduce a delay signal in control panel to prevent the tripping. Isn't that simple?

It is the only well said. I read all the answers of others and this one is the only correct you should communicate with the guy who wrote it.