Multiple steam boiler piping designs

tim smith

Just throwing out there to see if anyone has piping designs for multiple staged steam boilers that works well. I have seen the caravan system from slant fin but just wondering thoughts and possible ideas.
Thanks in advance
Tim

DanHolohan

Tim, I think it's best to pipe each boiler in accordance with the manufacturer's instructions, as though it was acting alone. From there, you pipe each boiler to a common header, sized to take the entire load. Your system takeoffs leave from that header.

On the return side, use a boiler-feed pump with either individual pumps for each boiler, or a zone valve to each boiler. Each boiler needs its own pump controller.

I would also have a spill trap on each boiler, mounted above the normal waterline and spilling back into the boiler-feed pump's receiver. That will keep the "off" boilers from flooding.

ScottSecor

For gravity returns we prefer this method (from Peerless TC manual);


For pumped returns (most of our jobs that require at least two steam boilers):

RayWohlfarth

I agree with Dan and Scott I have sold many multiple steam boilers and love them. I prefer using one feed water pump per boiler. The only drawback is if the pump goes down, so is the boiler. We have used solenoid valves and redundant boiler feed pumps but just make sure you have a strainer upstream of the solenoids. They have a way of plugging quickly.
The other issue I have found is controlling modular steam boilers. If you control it like a hydronic system (Staging one boiler at a time), you will not have the best results. The rads closets to the boilers will heat and there will be no steam for the far ones. What I found works best if the pipes are cold is to bring both boilers on at the same time. Once the steam pressure starts to rise, you could stage the boilers on and off.
Hope this helps
Ray

JUGHNE

I have done/service 2 boilers with a single pump.
For the new install I should have put the strainers on the feed line but didn't think of it at the time.
In addition of the balancing plug valves, I also added a bypass on the pump to put most of the water back into the tank.
Even the smallest pump supplies too much water and will kill the steam on a low water volume boiler.

And I have learned the control issue that Ray describes.
I first tried a 2 stage tstat but that is not good as the 1 degree difference would drop out the 2 nd boiler too early and not bring it back quick enough.

So now have both stages tied together and will consider cut out of #2 with rise with pressure.

The Steam Whisperer

I have found that if you keep the supply risers and manifolds and return manifolds generous, you don't have to pipe them as individual boilers. I.E. I installed 2xWM 380s with 4-inch supplies to a 4 inch common header and 2inch returns and return manifold. The boilers share a common condensate pump.
We can stage the boilers as needed to meet needs in the system. Think Mills Boiler.... essentially lots of smaller boilers all tied together through generous supply and return headers. Now if I was installing 2x WM1380s, piping them as separate boilers is probably prudent, since the pressure drop in the risers is higher. With the 380s with a single 4 inch riser, the steam velocity is so low that water can fall back into the boiler against the up flow of the steam. On like the Peerless 63/64 you should be able to safely go to 63-05 with a single 3 inch riser going to a common header.

nicholas bonham-carter

Would a Gifford loop, instead of a Hartford loop, (mounted above the waterline, on each), prevent the pressure from the active boiler from over-filling the inactive boiler?—NBC

RayWohlfarth

@nicholas bonham-carter ASME Code calls for a high level spill trap on commercial boilers.

DanHolohan

The off boiler floods because it's cooler than the on boiler. It acts like a condenser, and that's why the spill traps are so important.

RayWohlfarth

@DanHolohan Still teaching me after all these years

DanHolohan

And you me, Ray. Thanks.

The Steam Whisperer

Spill traps become unnecessary if the boilers are tied together with common return and supply manifolds of adequate size. Pressure is equalized on supply and returns and the water in the off condensing boiler just moves back into the common return and fills the on boiler. I've seen a number of old twin boiler heating plants set up this way.

JUGHNE

But only for gravity return systems, right?

EBEBRATT-Ed

ASME used to say that you cannot tie two boilers together below the water line without at least having check valve in the piping.

So as far as I am concerned spill traps are required.

There is no way to get rid of the condensed water without a spill trap

I have worked on plenty of Hydrotherm steam modular boilers that used to be the rage in the 80s. On steam a complete nightmare. had one with 16 modules

One boiler fires and builds pressure, it pushes water out the back into another boiler and the firing boiler goes off on LWCO. They were just tied together with a common suppl and return.


This question come up here often when people start talking multiple steam boilers. I just don't see it working unless it's done as @DanHolohan says.

But on the return side I don't see making it work without spill traps

I would like to see if you can hook two boilers together below the water line with out check valves and I do not think that is legal.

A leak in one boiler could dump the water from both boilers and I don't think ASME allows this

jumper

What I think I remember is that operators isolated off line boilers manually. Both in & out. This was in the days of atmospheric burners. I think some people even installed dampers in flues and safety officials worried that somebody will bypass safety and turn on fire while vent is partially dampered. Was it ever legal to restrict?

For operators 'twas a twice daily routine unless temperatures were expected to be well below freezing. Tepid radiators during working and sleep hours. Early morning and late afternoon overheat the building. Weekends were a compromise. Owners believed that they saved significant gas.

>>I have worked on plenty of Hydrotherm steam modular boilers that used to be the rage in the 80s. On steam a complete nightmare. had one with 16 modules

One boiler fires and builds pressure, it pushes water out the back into another boiler and the firing boiler goes off on LWCO. They were just tied together with a common suppl and return.<<

The Steam Whisperer

I could easily see the modules being a problem if they don't have really big supply and return headers. I've installed a number of common header 2 module steamers and they all keep the water lines stable. I really think it depends on the size of the headers ((and risers) effecting pressure drop between the boilers. The Mills boiler worked for over 100 years with common supply and return headers... all fired at once.

I've run them with both gravity and single pumped condensate returns. The pair of WM380s I recently installed are the most stable of all.... 4-inch risers and 4 inch header with a 2 inch common return header and each is fired as needed with 2 stage thermostat into a orificed 2 pipe system.

DanHolohan

I agree about the small modules. A one-ounce difference in pressure equals 1-3/4" of waterline. What are the chances that there's more than an ounce of pressure difference between the members of that module Rockette line?

The Steam Whisperer

Slant Fins are limited to 5 -300,000 btu input modules for each bank. I don't remember what Hydrotherm's guidelines where, but IIRC the modules were up to 400,000 btu with only a single 3 inch riser (43 ft/sec velocity). That's about double the steam velocity as on my little 380's (22 ft/sec), that have very little water carryover. IIRC twice the steam volume moving through a pipe will be 4 times the pressure drop and then add in the water drag.... not so good. I can see there being big problems if you stuck 16 of them in a single bank ( 4.8 million btu input), especially if you didn't have huge headers tying them together.
I have found that the Slant fins produce very dry steam ( that really fat end section with less boil and some internal baffles), so I would expect the piping pressure drop to be lower than the Hydrotherms.

tim smith

Thanks all, appreciate the feed back. We have had water level problems in the past with a couple caravan boiler we did. Slant fin layout. When one boiler is down. If I remember right, they never called for spill over traps and I think that's where problem lies. Sorry about the delay in responding to this. 
 Tim