5 replies to this topic

[aanita]

Dear Fellow

One of our heat exchangers steam trap by-pass line ( existing line size is 1") size to be increased by 2" to reduce steam velocity. Main steam trap line size is 3". Since during by-pass operation of the steam trap,(which frequently failure and leakinfg badly in flange) by-pass valve also start leaking. Steam trap is located outlet of the exchanger and inlet there is a temperature control valve(TIC). There is no flow control valve, however I guess there is about 2000 ~ 2500 Kg/hr of steam flow ( Saturated steam at 5 Bara pressure). What problems we may face if by-pass line size increase from 1" to 2".

Thanks in advance.

[Art Montemayor]

When you increase the condensate line size from 1” to 2” (I have to assume the customary use of schedule 80 pipe), you reduce the average velocity from 5.36 ft/sec to 1.3 ft/sec (based on 2,500 kg/hr of saturated condensate). This is OK. You have gone from a conventional velocity design figure to a more conservative value. This will increase the cost of the piping and valve and consume more space around the steam trap.

We can’t make a logical and meaningful comment on your process without you furnishing at least a detailed schematic flow diagram showing the locations and configuration of the exchanger, steam trap, by-pass piping, control valves, and instrumentation. Therefore, it is basically impossible to comment on future problems. However, your problem doesn’t seem to be fluid (or condensate) velocities. Your problem is making that silly steam trap work correctly, leak-proof, 100% of the time. If you can obtain a properly designed, procured, and installed steam trap, you should normally have no need to use the condensate by-pass circuit at all – except in an emergency to service the trap or drain the system. This is a routine, conventional installation of a steam trap that should not tolerate leaks or malfunctions. Something is wrong with the installation if you are frequently having to employ the by-pass circuit in order to allow the heat exchanger to operate. I would attack the CAUSE of the problem, not the EFFECTS.

[breizh]

Aanita ,
To echo the comments above, I 've added some information about steam traps .

Hope this helps

Breizh

[katmar]

The purpose of a steam trap is to allow condensate to pass while holding back the steam. This second part of its function (which I have underlined) is critical to the correct functioning of the heat exchanger. If the steam is not kept under pressure the condensing temperature decreases and there is less driving force for the exchanger. If a bypass is opened it will allow steam to whistle through the system and just get wasted, while the heat exchanger fails to do its duty. Increasing the size of the bypass will only make matters worse because there will be even less back pressure to ensure the correct condensing temperature.

In every plant that I have been involved with I have done my best to encourage the outright banning of steam trap bypasses. I prefer a drain valve before the trap straight to grade. Yes, it looks bad while the condensate is pouring on the ground and flash steam billows around but this makes it obvious that there is a problem with the trap and that it must be fixed. My approach is just like that explained by Art Montemayor above - fix the problem and not the effect. The "green" brigade might object to all this condensate being wasted, but in fact the bypass simply hides the tons of steam that are being wasted and the bypass is a much less green solution than putting the condensate on the ground for a short while. Having the condensate visible also allows you to throttle the drain valve while observing the quantity of steam being lost. There will always be more steam passing through than the trap would allow, but it does at least maintain some back pressure in the exchanger.

[kkala]

According to http://www.cheresources.com/invision/topic/11318-liquid-line-sizing/, post No 9, a line transporting up to 2.5 ton/h of flashing condensate (5 bara) would need a diameter of 2" . This size is expected to be OK downstream of the steam trap, provided that ΔP to the destination is adequate.
Suppose that the bypass line is used, in case that trap is temporarily removed for maintenance; max condensate flow should be 2.5 ton/h, plus some non condensed steam. In this sense bypass line should be (at least) 2". Of course there will be loss of steam during bypass operation, as previous posts point out. However the manual valve on bypass line can be tried in this rather exceptional case.
Armstrong does not recommend bypass in a steam trap, see http://www.armstronginternational.com/files/products/traps/pdf/installation/bult307.pdf.
Not having specific experience (bypasses are not sized by Process here), above may be a way out, at least in basic engineering. Bypass valve seems to play a more important role than bypass line size.
In case that steam trap condensate is directed to atmosphere (fertilizers, 1980), we had found out a suitable steam trap: http://www.cheresources.com/invision/topic/14186-steam-traps/page__p__56301__hl__+thermodynamic%20+steam%20+trap#entry56301.

Edited by kkala, 03 June 2012 - 04:54 PM.

[aanita]

Thanks a lot.