Design of Main Steam Header for Intermittent Service

[KernOily]

Hi guys. Looking for design suggestions here. I have the Spirax manuals and the Babcock Bible but I thought I'd ask here also.

I am designing a main steam header to intermittently carry 120,000 lb/hr of 65% quality steam at 600 psig. Line length is 3500 ft and it goes cross-country, up and down, across hill and dale. The line size we are using is 10" which will keep flow in the annular/mist flow regime. We design these lines to run in this flow regime because it prevents phase disengagement.

The problem with this particular line is that it will be in service (under full load) for only about 30 minutes out of every 24-hr period. The rest of the day the line sits idle. So, the short answer is, I need to drain the condensate from the line at the low points during the periods when the line is idle to prevent it from rocking and rolling when it's started back up.

My first thought is to use a pair of properly-sized thermodynamic traps at each low point in the line, one trap online and one standby, with the traps feeding a condensate receiver. Tricky part will be sizing the traps properly - we don't care if the condensate is removed or not while the line is in operation. I just need to drain the condensate when flow stops to ensure the line is empty when it's ready to be placed back in service.

What say ye? Any suggestions/ideas/brainstorming/insults/ comments of a generally obnoxious nature are welcome. Thanks guys!! Pete

 

[MJCronin]

Suggestions:

1) Find the actual steam flowrate at the destination based on the heat loss of your insulation system. Consider what will happen as the insulation degrades.

2) Perform a flexibility evaluation (computer based stress analysis) of the design.

3) Summarize and evaluate the capital cost of 3500 ft of piping, supports, traps etc....add the cost of maintenance and the cost of the energy and condensate lost over the life of the facility versus the cost of installing a 120,000 lb/hr (possibly used) boiler at the destination. You may be surprised at the present value of many years of energy loss.

4)Ask many questions and use well the fine and helpful people at Spirax Sarco.

My opinion only

-MJC

 

[steamdog]

Basically, you will have to drain the line at zero pressure so:
The traps MUST have gravity drain only. Condensate receiver must be atmospheric and physically below trap, which is also below the steam line.

If this is in the north, you may have freezing problems. There are drain valves available that open when the internal water temp reaches below 40F, you can use these also.

You may have to add vacuum breakers and air vents to the line. When the steam condenses it may draw a vacuum preventing the condensate to drain. The vacuum breaker will add air to the line and allow drainage, but then you must get rid of the air for proper steam temp/flow when you start-up again.

Final advice, think gravity.

 

[Jason5000]

Be careful with those drain valves that open at cool temperatures; if you use them, provide proper drains. If you just dump them to pad, they have the potential to make wonderful skating rinks.