lowering steam pressure in a industrial building

valdelocc

Facility was built back in the 1930,originally they had two coal B&W 60K LB/H boilers, time went by and like many others they outsourced part of the production and the steam load was reduced to a mere 20K to 30K LB/H, the coal boilers were scrapped, main pressure dropped from 150pisg to 90psig, now they are planning on cutting steam demand by getting rid of all steam driven production equipment and using the steam for space heating only, currently the steam travels about quarter of a mile from the powerhouse to AHU on the roof and perimeter wall radiation, there are several PRV reducing pressure from 90psi (main) to 12psi to loads. They want to drop main pressure to 12 psi, my first concern was the feed water pumps working outside the curb and the main was sized for high pressure may be too big for the low pressure steam turning it into water before it reach its destination. I would like to know what you guys think before advising the customer against it or perhaps it maybe a good idea and I just dont get the whole picture. thanks!

RJ_4

high press

Hopefully they are involving a mech.engineer     From my experience working with high press. steam,  smaller diameter piping sometimes could be used.   lowering the main press. with the same size piping may be a concern.

Pumpguy

Typically........

steam for building heat only, requires 2-3 psi steam pressure @ the various heat exchangers / radiators.  Of course ultimately this depends on what steam temperature they were sized for to put out their rated heat output. 



Vacuum pumps may help with steam distribution and overall system pressure differential.



As far as the condensate pumps are concerned, it depends on what discharge pressure they were sized for, and what they will see after all the changes are made.  They may require balancing valves on their discharge to keep them within their design operating curve. 



Some few decades ago, the Insurance Exchange Building in Chicago converted from high pressure to low pressure for building heat.  Large vacuum condensate pump sets were installed and these are still operating today.  To the best of my knowledge, no major supply or return piping changes were made.  Only consequence I remember were badly clogged strainers at the receiving tank inlets which had to be cleaned after a few weeks of operation.



After conversion, might be a good idea to check the strainers often, or let the condensate go to waste for a week or 2.



Feel free to contact me for more discussion.  Please see website below for more information.

jumper

I'd be concerned too

There's six times as much energy in the pipe at 90 psi as at 12 psi so a comparable heat loss from the pipe may condense some steam. On the other hand the steam goes faster so it spends less time in the pipe. So I agree with RJ that you need a mechanical engineer to calculate if the pipe can deliver enough heat at 12 psi. The condensate load depends only on heating demand, not on supply pressure. At least closely enough.

valdelocc

thanks

They have an inhouse mechanical engineer, he likes to oversimplify everything and then when things dont work they dump it on my lap, thats why I'm trying to gather as much pros and cons to inform the high powers before they spend thousands in the project.  

Dave in QCA

An Opportunity

This is not a simple situation but it sounds like there would certainly be an opportunity for huge savings and your engineer is wise to want to proceed with a plan to reduce the building pressure.



You used to require high pressure steam not only because the load was higher, but because much of the power or process equipment required that pressure to operate. You no longer have that requirement. You only have the heating load which has been described to be 12 psi. While most system can operate on pressure as low as 2 psi, others are designed at 5 psi or 10 psi. Control valve sizing, steam/air coil sizing, etc., may have all been calculated at the 10-12 psi range, so keeping that pressure may be necessary if that is the case. Also, may buildings with higher pressure systems (5 psi) have condensate lines that rise above the steam device, and therefore need enough pressure to accomplish that lift.



The critical factors for this project would be to calculate the full steam load of the building heat load and line losses in the building. Then, it should be a simple calculation to determine the capacity of the steam main, operating at 12 psi. Can it deliver the needed quantity of steam. There will likely be a little line loss under full load conditions, but I would guess that 1-2 psi loss between the boiler and the building being heated would not present a problem. 10 Psi will probably work just fine.



As for your feed pumps, you indeed bring up an important issue. Those pumps are designed to push feed water from a tank at atmospheric pressure into a boiler at 90 psi. If the pressure is dropped to 12 psi, they will operate much differently. They will pump water much faster. While the operation will probably be no longer on the performance curve, it will be above the curve not below. Cavitation tends to be a greater problem either at high temperatures or at very high heads where the pump is barely able to move the water. You will have the opposite condition. However, if the pumps in use are still being manufactured, it might be worth getting a manufacturer's rep to run the proposed operation of the pumps by the engineers in their home office and see whether a problem would be anticipated.



Good luck as this moves forward.

valdelocc

Thank you

Great explanation! The pumps are fed from a pressurized DA tank, feed water temp is 227F, my concern is that the pumps will over amp cause the lack of head with the new low pressure, it would be like removing the discharge piping and let them pump freely. Also the raiser and main drips traps were sized for high pressure, those are minor things although overlooked by the engineer, dont think it would be a big deal to replaced a hundred+ traps, what really worries me is the piping, we are taking miles of piping.  

Pumpguy

There are

special 2 foot NPSH pumps that are designed to handle high temperature condensate.  Frequently these can be fitted to existing receiver tanks. 



I would need to know the actual GPM and discharge pressure requirements to give you a specific recommendation. 



If your existing pumps are designed to discharge @ 90 psi, and this is changed to 12 psi, they will be way off their design curve, with the result that the volume discharged will be much higher, and that will result in an over amp condition.  You could throttle the discharges with balancing valves to put more head on the pumps, but I think there would be trouble with that much change in pressure.

Joe V_2

Interesting discussion

We used to operate at 130 psi but only out of ignorance. New building, engineers " told us to operate at 130 psi". I talked management into 90psi and that is were we run. we have around 30 AHU's, a two HEX HHW system and single HEX DHW system. The max normal steam pressure for these coils/HEX's is 15psi. I imagine when there is a power loss, of for any reason, all the control valves go wide open because of demand, there will be a slow return to normal at 12 psi. As for the feed water, Our dearator pumps have a return pipe with orifices' to the tank between the pump and supply line to boilers. I took out the Orifice and installed globe valves to regulate pressure to boilers. Was 225 psi; Is 155-160psi. Can drop to ten if I wanted to but Per an ASME code, the feed pump must be a percentage (3 or5) higher than the Boilers pressure relief valves. So, you are limited on pressure reduction, unless you replace your presure relief valves to a lower value

Joe V_2

The ASME Code says

...That the boiler feed water pump pressure must be 3% higher than the highest pressure releif valve setpoint plus any pressure drop between pump and boiler.  So, in my case, our relief valves pop at 150psi so:  150psi* 1.03 + 5psi= feed water pressure of 159.5psi. 

This prevents problems of returning to higher pressures and not being able to fill the boiler.