Maintaining Pressure in a Vessel 1

[goutam_freelance]

We are working on a system where water will be heated by steam. The steam condensate will be collected in a pressurised tank maintained at 70 psia. the condensate temperature in 266 deg F. The pressure inside the tank will be maintained by steam(at higher pressure) by a self actuating pressure regulator set for the vessel pressure. The condensate inflow is 26,600 lb/h which will be pumped by two(1W+1S) pumps back to the boiler to produce steam.

Does anybody have any experience with a similar system which is working. I want to have some feedback on your experience.

 

[quark]

Theoretically, this system will not work unless you have some plans apart from those furnished here.

For example, saturation pressure of condensate corresponding to 266F is 39.2psia. So how the condensate flows into a 70psia tank is a big question.

 

[goutam_freelance]

We have thought of that. Theoritically there will be a temperature gradient from liquid surface upwards(temperature will gradually rise) and the steam will gradually condense at the interface. However, the pressure regulator will admit steam from time to time to maintain the pressure, to compensate for the condensing steam.. The amount of condensation will folllow heat and mass transfer equations.

Any help on how to calculate?

Thank you.

 

[quark]

I am more puzzled now than before. Temperature gradient data is totally redundant here and I am not sure how it will assist fluid flow from low pressure to high pressure without any external assistance (condensate at 39.2psia to vessle at 70psia). Moreover, this will create grave problems to your heat transfer equipment.

Why do you want to maintain higher pressure in the condensate receiver? Suppose, if you maintain the vessel at 35psia, you will lose about 0.7% of condensate, i.e 186.2lb/hr. You have to exhaust that much flash steam from the condensate receiver and you are done.

 

[goutam_freelance]

I think the purpose was not explained in sufficient detail. The condensate tank pressure is maintained at higher than saturation to provide NPSH to the high pressure pumps(about 284 psia) feeding to a boiler. The flashing of steam also undesirable in this case as the make up to the main condenser will increase more than limit and so wii the oxygen in the main circuit as there is no deaerator in the system.

The auxiliary steam is mainly feeding to a water heater which heats heavy oil in a gas turbine in simple cycle. When the combined cycle is completed, the aux boiler will be shut down and steam will come from HRSG. The feed pumps will pump to HRSG in the latter case.

Hope this will give some insight to the little complicated system.


Thanks

 

[dcasto]

You do not describe the heated water system or the exchanger and how that is controlled, its all in the system. I always think about each end of the spectrum to get a real insight to such problems, something your process similator can't for you. what happens if no water is flowing, then you have no cooling and no steam condensing and full pressure on the system and no condensate with no NPSH. On the other end, more water than steam and you will condense all the steam nomatter what and the exchanger is flooded and the pumps cannot keep up.

There is an eqaulibrium point in the exchanger where the cooling water will condense the steam and you will have a level of condesate. If more water comes in, then you'll need more surface area and the condensate will drop, less water to heat, less area and a higher level. But you will always have full boiler pressure on the vessel, unless the steam goes through a control valve does it?

The easiest solution is to have the pumps under the exchanger low enough to get the NPSH you need if the exchanger had 1" of level in it. Then through level controls, control your temperature of the water out and not control the steam in (you could pressure control the steam to 70 psia if you want).

 

[goutam_freelance]

Thank you dcasto for valuable feedback.

The control aspect is handled with two control valves in split range control. The valves will throttle the steam when the heating duty is less thereby reducing the LMTD at the heat exchanger. The same control valve will close when the water flow stops.

Your suggestion for having a pump is very close. We had a lot of delibrations on that. However, we did not favour the same because of comperatively high temperature and our feedback regarding operating problems and high maintenance for these pumps. We sometimes have these pumps for pumping of regenerative feedwater heater drains in power cycle. In my observation, such service pumps is becoming rare now-a-days in power plants.

 

[dcasto]

I don't know the inlet steam temp or quality, but there isn't a big temperature drop on steam, 35 Deg F/100 psi drop on saturated steam. I wouldn't think at the 350F level a 30 F change in the steam temp wouldn't change LMTD.

But you can't close the steam valve because then the pressure will drop below 70psia. Look at how you control a flooded chiller.

The water outlet temperature is controled by raieing and lowering the level of the steam condensate. Need more heat in, less area is convered by condensate, need less heat in, then more tube arae is covered by the steam condensate.

 

[chicopee]

The best way to analyze your problem is to draw a single line circuitry of the boiler, load, condensate pumps and tank. With this circuitry develop a heating cycle diagram such as a T-S diagram. Your problem Goutamiam matches that of a feedwater heater. Without getting into the details that others have well presented, theoretically it should work as long as a feedwater pump is installed upstream of the tank to raise the pressure.

 

[goutam_freelance]

Sorry for being late. The following is information requested by dcasto.

Superheated steam is available at 428 F and 91.3 psia. However, after the Cv the pressure will be about 84 psia. The above are for simple cycle operation. During CC operation, the pressure after CV will be lass as the heat load is less(some preheating of heating water is done by HRSG incoming condensate water).

Any further observations?

 

[dcasto]

This is how I see it. The LMTD will only change 10% (or so). If the heat exchanged (Q) varies and Area is constant, the your only variable is the heat transfer coeff (U). The way U changes is to change the amount of area that has steam condensing (high U) versus hot water to hot water exchange. The pressurized tank should be right beside the exchanger and the level in it would be exactly the level in the exchanger. A verticle exchaner would be best. The top and bottom of the exchanger would be piped to the top and bottom of the pressurized tank.

You would not need to control the steam pressure as you stated unless there is a mechanical limit on the exchanger or pressurized tank. To control the outlet temperature, you would open a valve out of the bottom of the tank allowing the condensate to flow to its 70 psia user (the condensate should be slightly sub cooled so it should not flash but could under very low duty situations). If the temperature of the water leaving is to hot, the valve would be closed (or more closed) and if it is to cold the valve would open (be more open). In operation, we always had a low level shut off in this type of tank that would not allow live stream to exit. Of course if the low level switch closed the valve, the unit was overloaded and the you need to cut the water rate, live with cooler water, or get a larger exchanger.