Effects of Shrink or Swell Phenomena 2

[ghamsa]

We have 5 high pressure (HP) boilers producing steam at 625 psig and 720F. Recently, we commissioned 2 Heat Recovery Steam Generators (HRSG), which produce steam at 625psig and 720F. These HP boilers and the HRSG units are inter-connected on the supply side to the steam headers going to the users.

We in the process of purchasing a sixth HP steam boiler due to higher steam demand in the plant. But, we had a recommendation to install this sixth higher pressure boiler with larger steam drum. The intent of installing this sixth boiler with the larger steam drum is to absorb or mitigate the effects of shrink or swell phenomena whenever huge steam loads occur in the steam supply headers due to HRSG units’ trips in order to avoid unreasonable steam depressurization and pressurization rate in steam system.

I do not like this recommendation because I believe we should adopt a new instrumentation and control system to take care of the effects of shrink or swell phenomena.
please advise

 

[ApC2Kp]

ghamsa,
The larger steam drum for the added sixth boiler is probably the cheapest insurance against peak steam demands draining all the boiler drums, and exposing dry boiler tubes to excess heat. Most boiler failures are result of feedwater control failures to keep a minimum level in the steam drums. The addition of a sixth boiler is driven by steam demands, but it would also provide another layer of boiler protection.
The tricky part of drum feed water level control is due to the 'shrink and swell' phenomena as result of drum pressure drop with high steam demands. The water in the boiler drum and tubes have liquid flashing to vapor, so that the apparent liquid level rises temporarily, after a sudden demand has dropped the drum pressure. This temporary rise would normally reduce or stop the feedwater inflow, just when the opposite is needed, more feedwater flow. The instrumentation and controls might be configured in a more predictive manner - based on steam flow and pressure along with the level control. There are some problems to deal with during startup perhaps, when manual control could be used. The complexity of controls would be a problem for operators and maintenance. The extra volume in larger drum would give time for controls to respond to sudden demands.

 

[ghamsa]

ApC2KP

Now I understand what swell does to the boiler level. But dose "shrink phenomena" have bad effects on the boiler operations or safety?

or it dose not harm the boiler.

thanks for helping

 

[ghamsa]

In addtion to my previous questions,

How can I size the steam drums in order to "mitigate" the effects of swell and shrink phenomena?

Thanks for helping

 

[25362]

It may help to visit

http://www.spiraxsarco.com/learn/block.asp?id=3

in particular item 3.22 on steam accumulators and thread794-14016

 

[ApC2Kp]

ghamsa,
The 'shrink' phenomena in boiler drum level is result of a sudden decrease of steam demand. The pressure builds up in the drum as steam mass is stored with less outflow. The normal partial volume of vapor in the tubes is compressed, and cirulation may temporarily be interrupted in the tubes.
Drum level will temporarily drop, with level controls then increasing feedwater flow. If normal pressure is restored very quickly, the drum level may surge (swell) to exceed high level, resulting in liquid carryover to superheater, or even popping relief valve. The wide swings in feedwater flow might also affect the economizer performance with possible steaming conditions at its outlet.
One requirement for steam drum sizing might have a defined time of say 5 minutes, 10 minutes, or more of rated steam flow for the drum level to drop from high level to low switch level. The drum increased diameter might result in wall thickness of more than 2 inch, so drum length might be extended to some reasonable length, until practical size and cost limits are reached.

 

[rmw]

Ghamsa,

What type of feed-water level control do you have? I can't imagine that anyone would use anything less than 3 element control in this day in time, but I could be wrong. In my young days, there were a lot of single element where the only control element was drum level.

These are very susceptible to shrink and swell, and the level sensing instrument is sending the wrong signal to the FW level control valve, telling it to shut when the drum level swells, and it really needs water because mass is leaving the system in the form of steam and then having the valve pinched back or closed when the pressure finally equalizes and the level shrinks.

A two element control measures either incoming water flow or out going steam flow, and is a step in the right direction.

The real answer is a three element control which measures both steam flow out and water flow in and matches them with the level sensing element to trim it out.

With the steam flow measured against water flow in, by insuring that colder water is entering the drum, some of the 'swell' effect caused by the mixture of steam bubbles in the riser tubes is quenched by the colder water being pumped into the drum.

If you have a good tight three element control, it should be a good start. A larger drum is still going to shrink and swell, and is actually going to be harder to refill once the shrink occurs.

I'm for good controls. The larger drum sounds like solving a symptom of the problem, not the problem. Back pressure control valving might be an answer too. Just let the loads that are sucking the boiler dry suffer for a few minutes instead of tripping the boiler on low water which doesn't help the steam users one bit.

Note that I have worked around some plants that has vicious swinging loads, so I have seen my share of shrink and swell. It is one heck of a topic to try to teach to someone who doesn't understand thermodynamics.

Been there fought that fight many times.

rmw

 

[ghamsa]

Thanks all for helping out. We will have 3-element control in the boiler.

Again thanks for your help