sizing of steam drain lines

[Zehnder]

i am trying to dimension the drain lines of a power station steam piping system. what are the dimensioning criteria?
i would assume it is the condensation of saturated steam admitted to the cold piping during startup.
are there any thumb rules to design the drain system properly to ensure proper condensate removal?
the idea is to install a steam trap for continuous condensate removal during normal operation and a pneumatic operated valve for startup. an orifice upstream the valve shall ensure correct flow rate during startup. how shall this orifice be dimensioned?
thank you very much for your inputs.

 

[unclesyd]

You are not giving enough information, as designing a steam distribution system isn't that complicated, but it needs someone with experience to accomplish.

If you could come back with a little more information as to temperature, pressure, line size, length and mainly what’s on the other end I think someone will point you in the right direction.

 

[Zehnder]

ok. here some more information:
steam from two HRSG's (fired by gas tubines) is collected in a HP header and let to a steam turbine.
the steam parameters upstream ST are:
t=470DegC
p=35bara
flow=10kg/s (steam from 2 HRSG's)
there is also a steam extraction feeding a district heating condenser in winter time. the parameters of the extraction steam are:
t=185DegC
p=3bara
flow=9kg/s
the length of the live steam line from HRSG to the header (steam from 1 HRSG) is about 30m (diameter=150mm), from header to ST (steam from 2 HRSG's): 40m (diameter=200mm). length of extraction steam line: 35m (diameter=500mm).
my main concern is not the main piping itself but the sizing of the drain lines and the atmospheric drain vessel. i know very well the importance of properly drained steam lines after working many years as a commissioning engineer. so far, i never cared too much about the dimensions of the drain system but i have to admit, that i have seen bad dimensioned drain vessels and drain lines which caused lots of trouble. some practical advice would be very much appreciated.

 

[TD2K]

I know for our steam system (600 psig/750F) the drain legs were simply all opened to atmosphere and allowed to blow freely until that header was dry and up to operating temperatures. They were only closed AFTER the steam turbines were running and flow through the line was established to keep it hot.

During that phase, there was no attempt to even recover the condensate. I remember asking why (as I had just gotten out of school) and being told what liquid water does to rotating steam equipment and wasting some steam to atmosphere to heat up the line and make sure it was dry is cheap insurance.

I'm not sure what sizes the drips legs were, they weren't any larger than 1.5 inches and were more likely 1" piping. Yes, you blow a lot of steam out through those valves and it's noisy.

 

[unclesyd]

"TD2K" is right on the money about blow down after a shutdown. Failure to do this cost my company close to $500,000 within a 6 month period.

Our steam system is series of 650# circuits with letdown/desupheater stations (8) that drop the pressure to 300# and backup for 175#. All steam has a minimum of 30° superheat. The headers are all self draining. All auxiliary piping for some reason was set as 1 1/4" Sch 80. Most traps are installed with double globe valves above the strainer and a another valve at the header. The powerhouse system is augmented by 3 waste heat boiler generating 650# steam. As these systems are subject to a sudden dropout they are all connect to the main headers. These lines I would say have twice the number of traps and drains as the primary steam lines. Another twist all auxiliary lines in the waste heat system are 1" Sch 80. One system that I'm quite familiar with is a waste heat boiler generating 126,000 pounds/hr of 650# steam that has 40°F superheat. This system has 9 drains in about 2000' of line due to routing. This is the line that bit us with condensate.
It is easy to trap out a steam line, but it the blow downs and drains on start-up that get you.

Here are two site you might be interested in:

http://www.armstrong-intl.com/university/steamform.html

http://www.cuservices.net/index.htm

 

[PeteBennett]

Send me an e-mail address, and I'll send you the sizing charts we use here at Spirax Sarco, with a trap sizing booklet etc

 

[ssbei]

You should adhere to the following ASME Standard (the following is taken from the ASME web site):

"TDP-1 - 1998 Recommended Practices for the Prevention of Water Damage to Steam Tables used for Elec Power Gen

These recommended practices are concerned primarily with the prevention of water damage to steam turbines used for fossil fuel fired electric power generation. The practices cover design, operation, inspection, testing, and maintenance of those aspects of the following power plant systems and equipment concerned with the prevention of the induction of water into steam turbines and associated systems and equipment: (a) main steam system, piping, and drains; (b) reheat steam systems, piping, and drains; (c) reheat at temperating system; (d) turbine extraction systems, piping, and drains; (e) feedwater heaters, piping and drains; (f) turbine drain system; (g) turbine steam seal system, piping, and drains; (h) main steam attemperator sprays; (i) start-up systems; (j) condenser steam and water dumps; Any connection to the turbine is a potential source of water either by induction from external equipment or by accumulation of condensed steam. The sources treated specifically are those that have been found to be most frequently involved. have been found to be most frequently involved."

 

[zeven]

I used to size them using condensation energy :

Variables
Total weight of piping system (including valves etc)
Specific heat capacity of steel
Required warm-up rate
Energy generated by condensation of steam
Saftey factor to cover for heat loss through insualtion etc.

By using the above you can quickly get the amount of condensate (mass/sec) required to warm the pipes up at the rate required.

You can then use a reasonable flow velocity 3m/sec or 10 ft/sec coupled with the number of drain lines to size the line.

Couple of points to be carefull of:

Ensure each individual drain is not overloaded, or if so, is increased in size to take care of the extra duty.
If the required Superheat is more than a few degrees then use bypasses to achieve the last part, as you will destroy the drain valves passing high velocity 2-phase flow steam/water.

 

[Paperpete]

Having worked in papermills for the last 10+ years and been subjected to some "real time engineering sollutions" here are my two cents.

Not only do you need large enough drains but also drains in the right places. I follow these rules of thumb and have designed many troublefree return systems.


Whenever up from a horiz. line to a vertical line use a Tee instead of an elbow. Run the line full size down from the tee a foot or more. Reduce down to your trapsize. Put another tee and then the trap in the small horizontal line. Continue the other small line down to your drain valve. This way dirt will go to the drain valve and not clog up your trap.
When there is additional need for traps due to a long horiz. line then I use a tee in line aiming down and then use the same procedure again with the reducer-trap-drain.In one large system that I put the customer wanted a "warm up button" the way I designed this was with a small steamvalve putting steam in the line untill hot condensate was detected at the drain furtest away AND the header was up to 3/4 of working pressure. That worked great in that application. I generally believe that the safest systems are where people who understands steam are operating and they perform the warmups themself. It's back to the old MAN or MACHINE question.

 

[corners]

Since this is a power plant, stay within the guidelines of TDP-1. I do NOT recommend using a Spirax Sarco sizing chart to size power piping drains. They may not meet code.

TDP-1 is very straight forward in drain sizing.

 

[Zehnder]

many thanks to everybody for the very usefull information. in a first step i'm going ahead with the heat balance estimation model from "zeven".
i already ordered TDP-1 to be on the safe side.