check valve / non-return flap valve after the turbine

[Pederator]

Hello All.

Consider following situation: we have energy conversion system of municipal waste incineration plant (or coal fired CHP). The turbine has its bypass. After the turbine, before connection with bypass is a steam trap. Now it is a question, if before / after the steam trap should be installed a check valve / non-return flap valve (i don't know exact name), to prevent back flow from the bypass (if it is used). I'm not sure if the back flow to the turbine is possible to occur, since in a condenser is a very low pressure.

Guys, what are your opinions about this? Thanks in advance for help.

 

[Pederator]

I'd like to admit, that staem trap is used only if it is an air condenser based system, if there is a water cooled condenser, we have a flash tank instead of steam trap.

 

[gerhardl]

All steam systems must be considered in its whole: total piping layout, with pipe courses (horizontal,vertical or slated). Valves, filters, steamtraps etc. to be selected and installed after consideration of all pressure and fluid situations possible to occur.

A steamtrap is constructed to let condensed steam (water) out of the system, and will close by hot steam before the steamtrap, regardless of type (mechanical, hydrodynamic or termostatic). I have difficulties seeing a 'standard' steam system where higher pressure could occur downstream a steamtrap than upstream under normal circumstances. (Higher pressure would require higher temperature downstream.)

Backlash into the system from before a steamtrap is not normal, even if you could imagine condensate overflow before the stamtrap. This would indicate wrong layout, dimensioning, or non-functional steamtrap.

Isolation valves are however normal before and after steamtraps.

Selection of steamtraps and dimensioning and layout of steam systems should be done consulting experienced specialists. All solid suppliers and distributers will be able to assist you.

 

[Pederator]

You did not understand me. The case is not a steam trap or flash tank, but a check valve / non-return flap valve. This valve (or flap) would prevent the back flow of a steam from the bypass to the turbine - if bypass is opened. But - like I said - I am not sure if this back flow can occur since we have a very low pressure in the condenser.

 

[gerhardl]

OK!

The general answer is the same: 'All steam systems must be considered in its whole: total piping layout, with pipe courses (horizontal,vertical or slated). Valves, filters, steamtraps etc. to be selected and installed after consideration of all pressure and fluid situations possible to occur.' Likewise all comments on steamtraps and checkvalves are valid.

As you, and as cited above, I cannot see a situation where a higher pressure could occur downstream steamtrap and bypass in a normal layot, and normally a checkvalve should not be necessary or recommended.

However, I understand your wish to protect the high-cost turbine, and should have liked to see the total piping layout with dimensions and all fluid data.

A checkvalve must be a suitable type, dimensioned and oriented not to interfere with the condensate flow. If the checkvalve is placed near turbine (ideal for protecting turbine from backlash) one must be 100% sure that condensate can not gather between checkvalve and turbine (for instance under long stops). This would be against draining requirements and could increase possibillity of flashing/backlash and waterhammer. Same for other placements.

I suspect your system to be of larger type, but below brands might have information of interest for you.

http://www.spiraxsarco.com/pdfs/SB/p126_01.pdf

http://www.flowserve.com/Products/H...dIndustrySearchTerm=&prodIsSubmit=true&page=0

 

[Pederator]

Thanks for this valuable reply. In the meantime I figured out that such a check valve / non-return flap valve would be necessary if the system must operate when turbine is under repair or overhaul. In that case the system of course is working on a 100% cogeneration mode. You must keep in mind that turbine outlet diameter is very large, in my case about 2 meters, therefore the valve would be very large, so I am wondering how to resolve this. Another thing is that this one valve, from safety and robustness reasons, wouldn't be sufficient. I have no idea if such a solutions exist in practice in a CHP plants fired by coal, waste, biomass etc.

 

[pennpiper]

Pederator, You wrote: I figured out that such a check valve / non-return flap valve would be necessary if the system must operate when turbine is under repair or overhaul.
It would not be safe to do overhaul, maintenance or repair on one part of an active system and rely on just a Check Valve (Non-Return Valve) for protection.

Pederator, You wrote: In that case the system of course is working on a 100% cogeneration mode.
It does not matter what mode it is, the system is operating and unsafe for maintenance.


Pederator, You wrote: You must keep in mind that turbine outlet diameter is very large, in my case about 2 meters, therefore the valve would be very large,
Oh! Now you tell us the size. The size makes a big difference in our thinking out here and definitely has a bearing on the answers you will get. I do not think you will be able to find a 2 meter Check Valve as a standard off-the-shelf item.

Pederator, You wrote: Another thing is that this one valve, from safety and robustness reasons, wouldn't be sufficient.
You have got that right.


Pederator, You wrote: I have no idea if such a solutions exist in practice in a CHP plants fired by coal, waste, biomass etc.
What research have you done?

prognosis: Lead or Lag

 

[Pederator]

I have done some google research. All in all, I bet that check valves are not in use for that application, and in case of turbine breakdown / repair / overhaul all the system must be shutdowned. Am I right?

 

[gerhardl]

Predator: to your last posting:

Yes, or the turbine must be safely (note: safely and allowed!) isolated or disconnected from the rest of the system. How, and with what equipment, depends on piping system and layout details, rules and regulations. Several possible options might be discussed.


a) Complete shutdown of total system
b) As a), but in addition turbine physically disconnected (removed), connections secured by readymade blindflanges. (Do you actually have to operate the steam system without the turbine, and is the capacity the same or considerably less then, able to be operated on reduced pressure/dimensions by bypass and separate pressure reduction station for such cases???? etc. etc.)
c) Isolation valves for turbine on upstream, highpressure steam connection with one or several (for safety) isolation valves, considering theese to be considerably less than downstream 2000mm. Downstream from turbine open/disconnected, and/or condensate piping for regenerating from turbine disconnected/isolated. Rest of system run reduced bypass and pressure reduction station

... and several others.

As you see, the more you go into details, the more questions and sub-details must be answered and solved.

I am again landing on my first advice: Selection of steamtraps and dimensioning and layout of steam systems should be done consulting experienced specialists.

Steam efficiency could be vastly different if layout have only a few wrong details, and in addition perhaps danger of of explosion and waterhammer.

Good luck1