Sudden Drop of Feedwater Pressure

When the Pressure drops it is caused by a REDUCED FLOW or REDUCED FLOW RESISTANCE.

This could be from another flow path opening and using more flow than the pump is capable of.

Is the coupling scoop tube controlled manually or automatically? If automatically, how? What condition are the controls in?

Is this a new installation? Or a new problem with an old installation?

What is the condition of the turbo coupling? Condition of the oil?

Was the coupling recently overhauled / worked on?

I can't see the pump directly at this point, there are just too many things in the way.

There are several reasons you might be getting this:

1. Your control system is tuned improperly. Check to see that the pump is not being commanded to drop the pressure.

2. Check the feed lines and tanks. You may have an NPSH (net postive suction head) issue. If your feed tank levels are dropping, you may not have enough suction head to prevent cavitation. Cavitation tends to do bad things, including making a pump ineffective. If they are dropping too much, you could be sucking air in the line. Water pumps tend not to be great air pumps.

3. Check the pump vibration. Do you notice an increase in vibration when this happens?

4. Check the pump speed and amperage, does it go up or down in this adventure. I'll bet that it doesn't stay the same.

5. Check the feed system to make sure that the temperature of the incoming water is not rising. Why might it rise? Perhaps you are dumping hot water into the boiler feed water, or perhaps the pipe temperature is going up due to friction.

6. For that matter, check the pump temperature. If it is rising, you may have increased clearances, which decrease pressure.

7. Check where the water is going. It would be interesting to discover that just about the time you need water from that pump, some other valve is opening to relieve the pressure or use water for another process.

8. Check the pump seal under pressure. You might be dropping all your water out the wrong end of the pump.

9. if it is really a problem, send someone (several someones) to watch each step of the process from beginning to end and report what they see, hear, and feel. Observations such as a vortex in the tank (not good), overheating pump, vibration, broken pump seal, banging pipes, leaks, odd noises, the smell of ozone or burining, etc.

Good luck,

Michael

What is your feed water temperature? What is the height of your feed water tank?

What is your boiler operating pressure? What is your boiler capacity?

For temperature 90C & + the height must be minimum 4.5 metre to safeguard against pump cavitation. The failure may have been due to this that your pump has cavitated due flash steam formation above 90C resulting in imploding upon the impeller blades and damaging them for the pump to lose pressure.

For future safety :

1 . Install a Flow Switch (ITT FS4-3) on the pump outlet to warn you.

2 . Install a temperature sensing at the exhaust & Controller to cut iff your burner in event this pump failure occurs and even the lo-lo water cut off fails.This will safeguard you & your boiler.

It sounds like your instantaneous demand is sucking the water and pressure from the boiler and causing it to trip on low water. If the boiler pressure drops low enough it may appear that your feed pump is dropping pressure when in reality it is pumping into a depleted system. Further, if the feedwater valve is of the motorized actuator style, it may be too slow to react and by the time it opens fully it is too late.

There are 2 potential remedies in addition to the previous postings. First would be to provide a wet steam accumulator (AKA, surge tank) between the load and the supply. These are designed specifically for instant demands which can overwhelm a boiler in a moment, but can easily allow a small boiler to supply steam on an average basis. Second, you may need to install a back pressure valve between the boilers and the load which will sense the lowering system pressure and close on drop to protect the boilers from overload. The following link has lots of good information on steam surge tanks.

http://www.spiraxsarco.com/

A very good answer. This is normally encountered mostly in a new system setup and wrong engineering. These will also be common on boiler priming particularly on high loads with an on/off level controller. Ideal solution :

1 . If old system and in past this problem did not occur check pump for cavitation.

2 . Install modulating level controlling system.

3 . undertake your 2 potential remedies as additional safety.

Gentlemen,

From the real time trending:

1.Pump amperage has gone up from normally around 58 amp to about 68 amp.

2.RPM of the turbocoupling/pump/motor remained the same about 2900.

3.Scoope tube position remained at 100% as we select it to manual mode.

FYI,on the discharge side there is a SCHROEDER valve(3 way N/R valve)which we renewed recently and the centrifugal Feedwater pump has been serviced few months ago.

We haven't checked the feedwater tank/feedwater temperature yet but I bet it remianed the same.Anyway I will check the trending again.

Until now the problem not yet solved.