boiler steam accumulator

[larry_s]

In the boiler steam accumulator setup, there are two valves. One is a proportional valve that controls high-temperature steam entering the container to heat the water in the equipment, and the other is a switch valve that controls steam entering the container to increase the pressure in the equipment. When the water temperature is below 145°, and the real-time temperature of the water corresponds to a saturation pressure less than the equipment pressure + 0.05mpa, the switch valve opens to pressurize the equipment; otherwise, it closes. So, what's the reason for this pressurization?

 

[crshears]

I've had a look almost all the way to the bottom of the page here https://www.spiraxsarco.com/learn-about-steam/the-boiler-house/steam-accumulators?sc_lang=en-GB and I'm not sure which valve in which diagram you're calling the "switch valve".

Different terminology in use, or am I missing something?

 

[georgeverghese]

The info in the spirax sarco link crshears attached doesnt show controls, but there are some clues in the narrative, so here's my guess:

a)The proportioning valve soe the job of steam accumulation and is based on temp control of the liquid phase in the accumulator - so this valve must be feeding the steam injectors - this function is active only when "excess" steam beyond normal continous demand is available from the boiler, so there may be a backpressure valve upstream of this valve that tends to close. So this function enables excess steam to be generated by flashing high temp liquid to meet peak demand, which is beyond the flow capacity of the pressurisation valve.
b)The pressurisation valve feeds the vapor space of the accumulator and is active all the the time, but is by some mechanism high flow limited to normal max

 

[larry_s]

The info in the spirax sarco link crshears attached doesnt show controls, but there are some clues in the narrative, so here's my guess:

a)The proportioning valve soe the job of steam accumulation and is based on temp control of the liquid phase in the accumulator - so this valve must be feeding the steam injectors - this function is active only when "excess" steam beyond normal continous demand is available from the boiler, so there may be a backpressure valve upstream of this valve that tends to close. So this function enables excess steam to be generated by flashing high temp liquid to meet peak demand, which is beyond the flow capacity of the pressurisation valve.
b)The pressurisation valve feeds the vapor space of the accumulator and is active all the the time, but is by some mechanism high flow limited to normal max

'Through high-temperature steam heating the water in the container, increasing its pressure and temperature to maintain energy, the water temperature in this container is below 145°, so high-temperature steam is needed to pressurize the equipment through valve b.

The idea is: calculate the saturation pressure value Psat based on the actual water temperature +5°. When Psat is greater than the actual pressure value of the container and the actual pressure value of the container is less than (Psat + 0.4bar), valve b opens; otherwise, it closes.

By heating the water through valve a and pressurizing the container with valve b, the water temperature reaches above 145°. At this point, there's no need to pressurize through valve b, just heat the water through valve a.'

If we don't check the pressure value, will it cause water hammer because of the high pressure from the large amount of steam on the water surface?

 

[pierreick]

Hi,
You may want to consult those guys:

https://www.bosch-industrial.com/ocsmedia/optimized/full/o348968v272_BR_BoilerHouseComponents_en.pdf

Pierre

 

[Dhurjati Sen]

Is the steam accumulator connected to a deaerator?
Could that explain the increase in pressure?

 

[larry_s]

Is the steam accumulator connected to a deaerator?
Could that explain the increase in pressure?

The steam comes from the boiler drum.
The steam produced by the steam generator is divided into two outputs: one heats the water in the steam accumulator to raise its pressure and temperature, and the other is used to pressurize the accumulator.

 

[crshears]

Hmmm . . . not sure why the "switch valve" would necessarily be required; the lit seems to indicate that the normal steam supply to an accu is always via the injectors, with blowdown via an economizing heat exchanger used to reduce the water level in the accu if it gets too high.

I suppose it all comes down to thermodynamic balance . . .

 

[georgeverghese]

I see the logic in the controls you have now (?) or what you propose, but I dont see why you need to complicate it so much - based on your proposal , you've got to have an algorithm in the process controls to calculate Psat based on temperature of the liquid phase. Why bother with this complication when all you need is a plain flow limited - forward pressure control through valve (b) ? I dont see a problem with this control valve operating on-off on pressure gap as a switching valve, if that is how you prefer it to be.

When liquid steam flashes here, flash steam generated may overwhelm the vapor handling capacity of this acc - do you have a wiremesh demister or similar on steam exit here ? Secondly, how do you high limit the flow of total exit steam from this accumulator when there is a transient peak flow demand ? Do you have a flow control loop on steam exit so that this accumulator does not generate too much flash steam, or is there a backpressure regulator on steam exit ? Uncontrolled Flash steam generation would obviously be very violent.

If there is condensate carryover into the flash steam leaving this drum, of if there are low point legs in this steam distribution header piping where condensate can accumulate, condensate hammer can occur as steam shock cools down into liquid upon contact with cold condensate. All low point piping legs should be trapped for condensate removal.

 

[georgeverghese]

"The idea is: calculate the saturation pressure value Psat based on the actual water temperature +5°. When Psat is greater than the actual pressure value of the container and the actual pressure value of the container is less than (Psat + 0.4bar), valve b opens; otherwise, it closes."

You mean valve "a" opens for injecting live steam into the water, not valve "b" ?

Where did you find this criterion? This is a rather narrow range of operation for valve "a".

 

[larry_s]

"The idea is: calculate the saturation pressure value Psat based on the actual water temperature +5°. When Psat is greater than the actual pressure value of the container and the actual pressure value of the container is less than (Psat + 0.4bar), valve b opens; otherwise, it closes."

You mean valve "a" opens for injecting live steam into the water, not valve "b" ?

Where did you find this criterion? This is a rather narrow range of operation for valve "a".

There is a temperature controller (PID) in the steam accumulator, this controller is set to 156°, the steam accumulator starts in the cold state, the temperature of the water is low, so this PID is gradually output until 100%, this opening is used to control the proportional valve (picture a) to control the steam flow into the accumulator to heat the water, Accompanying calculate the saturation pressure value Psat based on the actual water temperature +5°. When Psat is greater than the actual pressure value of the container and the actual pressure value of the container is less than (Psat + 0.4bar) ，If this condition is met, then the b valve opens.

If the water temperature reaches the set temperature of 156°, then the cold start is complete, and there's no need to pressurize valve b anymore; you just need valve a to heat the water in the heat storage. Simple idea: during the cold start, you need valve b to pressurize and valve a to heat the water; once the cold start is done, you only need valve a to heat the water.

I don't get why we need the b valve to pressurize during a cold start?

 

[larry_s]

"The idea is: calculate the saturation pressure value Psat based on the actual water temperature +5°. When Psat is greater than the actual pressure value of the container and the actual pressure value of the container is less than (Psat + 0.4bar), valve b opens; otherwise, it closes."

You mean valve "a" opens for injecting live steam into the water, not valve "b" ?

Where did you find this criterion? This is a rather narrow range of operation for valve "a".

I saw this in the boiler PLC program.

 

[georgeverghese]

Okay, what about this :

When Psat is greater than the actual pressure value of the container AND the actual pressure value of the container is less than (Psat + 0.4bar) ，If this condition is met, then the b valve opens.

This open logic for valve b makes sense to me if AND is replaced with OR

Valve b opens during cold start pressure the accumulator vapor space so as to feed consumers ? Presume that during this time, valve a would only be open to heat water only if pressure in the accumulator has exceeded a predetermined value, ie consumption is less than max boiler generation capacity ?