Pump power for a steam to hot water system 1

[MikeL2012]

Hi,
I am not designing a system but doing a feasibility study (rough estimate) and not that versed with real system design.
Currently the district heating system has 2 steam boilers which supply steam to mechanical rooms in the campus. There are lost of steam leaks and the steam stystem needs to be replaced . one of the options considered is - convert to hot water heating system.
I am adopting the following approach (please excuse my ignorance of design and any help/suggestion will be highly appreciated).:
I have 2 boilers: output capacity 57,000 Kbtu/hr each
GPM for one boiler (assuming delta T 50 - average based on the measured data from current building operation): 2280 GPM
for 2 boilers : GPM = 4560

I was hoping to use eh following eqn for Pump sizes provided know the pressure drop.

Minimum Horsepower Required = ((Max GPM) X (Max PSI) / 1710) / (Efficiency in Percentage) X (Specific Gravity of Material)

Currently I have 41920 meter (26 miles) of high pressure steam line (20,960 m for steam line and 20,960 m of condensate return line).
how do I calculate the pressure drop for the pump sizing requirement (for converting it to hot water line)? any help/suggestion will be highly appreciated..
thanks,

Mike

 

[willard3]

Steam piping sizing is different than water piping and any pressure drop guess would be a bad one.

Water piping design for campus systems is 4' head loss/100' piping. In larger systems, system is usually limited by velocity and should be below 8'/second.

You will do better to hire an Engineer familiar with piping.

 

[Drazen]

i would assume that you want to do basic economic study, not detailed engineering, because in case of engineering work you would need much more than simple explanations of basic calculating equations, which is far beyond any level of tip-talk.

so: i would never replace steam line with hot water line only because of leaks. if you have leaks that could mean lines are obsolete or were improperly installed, so they need to be repaired or replaced.

if you do economic study, you must have knowledge of real consumption. high pressure steam can transport much, much more thermal energy than hot water, but if level of consumption allows it, hot water can still be feasible.

feasible because steam line has an issue of condensate utilization - how much of it is returned, and at which temperature.

you need to calculate heat losses for both scenarios, and pumping electricity consumption for both scenarios.

groundwork costs need to be estimated as well - what do you have currently, above ground or underground lines and what would you plan in future.

than, you would possibly need to replace heat exchangers at consumption points. of course, you need data to be able to estimate whether such activity is needed or not. you need to collect data on exchangers for that purpose.

further on, you need primary heat exchangers that will convert steam from boilers to hot water. how much would they cost? can they be placed in existing boiler plant? estimate of available space is needed.

as you might notice, data on pump size is far less relevant than many other aspects. for economic calculations, you should not size pump, but look at average pressure drop of design lines for district heating and simply multiply it with your lengths.

on top of all, i believe you could possibly realize that you would need professional help for that, both you and manager who forces you to do "rough" estimate can sink into big trouble if at least all critical aspects were not properly analyzed (and you can notice there are number of such aspects).

 

[MikeL2012]

Thank you Drazen and willard3 for your insights. This exercise is just for a feasibility study. This option will not make sense in this project but it is needed to complete the exercise. I agree pump is a small part of the whole project. All other features (boilers, heat exchangers etc) of this options were worked out by someone else but he had to leave for Christmas break and the report must go before the end of the year and that person will not be back before Jan 2. The pressure drop in the steam system (measured) was 160 psig. Do you think I should use this pressure drop instead of calculations based on pipe length, gpm and pipe dia?

Mike

 

[willard3]

>>>>>>>>>The pressure drop in the steam system (measured) was 160 psig. Do you think I should use this pressure drop instead of calculations based on pipe length, gpm and pipe dia? >>>>>>>>>


In short, no.

 

[MikeL2012]

As Drazen suggested 4' head loss/100' piping; my head loss would be enormous. For 41,922 m (135,472'), It would be 5419'. Does it seem reasonable? the measured pressure was 160 psig (368 foot H2O). sorry for these novice questions but any help to solve my problem will be greatly appreciated.
thanks
M

 

[Drazen]

Steam pressure loss cannot be compared with water pressure loss in pipes, I can only strongly suggest that you give up that idea.

I hope you do not plan to simply put pump on steam piping which came to my mind while looking at previous question). If that is the case, first you need to do is to walk on the field, and some common sense issues will appear by itself []

 

[MikeL2012]

Thanks, Drazen. I appreciate your help. I think I will rather wait for one week that doing wrong calculations.

 

[KiwiMace]

Large scale water distributions tend to be configured as primary production at one or more boiler plants, secondary main distribution (transmission) as a campus loop, and tertiary branch distribution delivering the right amount of water at the required head to each building. Each loop with its own pumps.

Steam is produced at a pressure which ensures its delivery at the required pressure at the point of use in a trunk and branch network, with separate condensate return.

These are fundamentally different in configuration. You entire piping network may need to be replaced. For a rough calc, you need a rough idea of the point of use heat loads, then work out a rough flow based on your design delta t, then size your pipes per hot water. If there is no significant reuse in the new and existing sizing, then sketch a new layout and propose that in your study.

 

[willard3]

This is the kind of question managers ask assuming there is an easy answer.....there isn't one.

You will do this feasibility study and use wrong numbers to arrive at the wrong price. The manager, thinking this will make him look good, will authorize the project when you will find out what was wrong with the approach in the first place and everybody will be sticking pins in your doll.

This is an involved project and, if you need pricing, assume that the whole steam system will be removed (terminal units, HX, piping, controls, coils, valves, piping and stc) and replaced by a hot water system. There is no way to sugar-coat this for your manager.

 

[urgross]

I've worked on similar projects before and Willard is spot on. Pretty much everything, down to steam heating coils to all distribution would need to go. In dealing with 1940's era steam distribution asbestos was also a major consideration for cost and schedule. After you estimate how much old steam and new heating hot water distribution will cost, then comes any process or motive steam equipment to be replaced. Operating at lowest possible pressure, improved condensate management, and energy recovery ended up being the lowest LCCA. Another basic consideration is what you pay for electricity versus natural gas or #2 fuel oil, and whether a redundant power source is needed. Hard to compete natural gas against electricity in the region I'm in.

 

[cry22]

You mean your senior buddy on vacation till jan 2nd cannot asnwer your question from his retreat? no access to e-mail? where do you work at?
I answer quations from my blackberry on travel instantly from half way around the world.

Back to your question
The pressure you read is the steam pressure at 160 PSIg, NOT the pressure drop through the steam piping.
Bet in such a wide campus system, there must be need for steam in the campus anyway, labs, sterilizers, etc.
As an option, you should be looking into having a HW boiler plant (using high efficiency condensing boilers for example) at each building instead of main HW boiler plant Vs steam system retrofit.

if you want to compare HW boiler Vs steam campus-wide, you should look into High temperature water system (250F+) with 100F temperature drop. Of course with HX's rated at high pressure in each building etc.

You could also look into replacing all the leaking expansion joints in the lines (assumming they are accessible in a tunnel or manholes), but you need lots of downtime on the system, you campus must remain in operation and if this is a hospital, you cannot have downtime on steam supply.

It looks like you're not qualified to evaluate the right options or the right approach to begin with. There is no quaetion that you should not be doing this task - even if it is just a study.

As others have said, get help.