Proper boiler control of a large STEAM system (16,000,000 btu)

Mike_Sheppard

Good evening,

I want to get some of your guy's opinions on how you would control this system. This is a large building from 1929 in DC and it is currently a bit of a wreck.

I am about to install a new vacuum pump tank system to replace the failed tank, it is a 6 figure job.

Info:
- Two pipe steam system
- Radiators have thermostatic traps
- Ends of mains have F&T traps
- (2) 8,000,000 btu boilers
- Each boiler is fully modulating with a 2:1 turndown. 8 million btu max and 4 million btu minimum

How the system is controlled currently:
- Heat timer MPCQ on the wall.
- Instead of using a temperature sensor to "establish steam" this controller uses a pressure sensor. There is a pressure transducer in the main boiler header that senses the pressure and when it reaches the setting (1.5 psi) it knows steam is established and begins it's cycle.
- The Heat Timer MPCQ does NOT turn the boilers on and off based on that 1.5 psi setting. All the heat timer is doing is using 1.5 psi to establish steam, it does not have the ability to turn the boilers on/off based on pressure or the ability to modulate the burners.
- The boilers have their own operator pressuretrol and modulation pressuretrol. They are using a standard 0-15 psi pressuretrol for operator and for modulation. No modulation control exists that can operate in the 0-2 psi range to modulate the burners. And the existing pressuretrol 0-15 range is too high. The system currently gets up to about 5-7 psi while operating.


What would your guy's suggestions be for controlling the pressure in this system? The only Heat Timer controller that can directly control modulation of the burner is the Multi-Mod platinum but that one lacks the ability to do steam cycle outdoor reset.

I was thinking the simplest and most cost efficient solution would be to leave the existing heat timer and add vaporstats to the boilers. I would use a 0-4 psi vaporstat for the operator set to 1.5-2 psi, and instead of using a Honeywell modulation pressuretrol I would just use a 0-16 ounce vaporstat to control high-fire / low-fire of the burners. Instead of full modulation they would just be either low fire or high fire. Burners would fire up and go to high fire, once reaching around 10-12 ounces go down to low fire, and back to high fire if they drop below 10-12 ounces again.

One other possible option I was thinking of was adding a second heat timer, the Multi-Mod, to control modulation of the burners to a setpoint of say, 1.5 psi, but also leave the Heat Timer MPCQ to perform the 60 minute steam cycle of the boilers based on outdoor air temperature.

Steamhead

First thing I'd do is change the steam-established sensor to a temperature type. Then you can lower the pressure as much as you can.

retiredguy

I have been retired for 15 years so my comments are from that period. Why is there just a 2 to 1 turn down on the new boilers. In the 1970's the standard turn down was usually 3 to 1, which later became 5 to 1 and sometimes 10 to 1 depending on the equipment selected. The boiler's steam supply pressure was set by the system design engineer. I can't recommend a type of control since things have changed a lot since 2007, but I am sure you will get a lot of excellent recommendations.

Mike_Sheppard

@Steamhead I agree. I already scoped the basement and found a decent location on the other side of the building, almost all the way to the furthest steam main. But I’m still left with the same problem - the Heat Timer that establishes steam off of temperature still cannot cycle the boilers on pressure or control their modulation.

@retiredguy I don’t know why they were spec’d with only 2:1 turndown. It may be because they only run oil and these specific burners can’t do much more than a 2:1 turndown on oil from the nozzles. They were installed in 2008. Cleaver Brooks FLE boilers with Cleaver Brooks Profire burners.

EBEBRATT-Ed

Tek mar has a good steam control maybe look into that. I think it will work better with the steam established sensor at the far end of the steam main wherever that may be, It might be at the riser at the top of the building. Having the sensor in the boiler room does no good

Once the steam established sensor is satisfied that steam is established it starts timing the burner on period based on the outdoor temp.

Mike_Sheppard

@EBEBRATT-Ed I don't need a cycle control. I need a way to control the boiler modulation and pressure.

The only good solution I can think of is using a second heat timer that can do modulation and use a steam pressure sensor.

EBEBRATT-Ed

Mike attached is my home made lead lag system for two boilers. It uses a double pole switch for each boiler (this could be a spdt relay (like a RIB) to change each boiler from lead to lag. With this you can run both boilers on lead, both on lag or 1 lead and 1 lag. This can be done with a time clock and a couple of rib relays. I would add two header mounted (common header) pressure controls to do this. That would take care of the operating pressure.

As far as modulation goes 2 vaporstats (1/boile) operating a rib spst relay. Disconnect the existing modulation control. You would take R from each boiler to the common of the rib, NC contact to white (drive to lo fire) NO to blue so with the vapor stat open the rib drops out and your drive to lo, vapor stat closes rib pulls in makes R-B drives to high.

Alternative put 1 vaporstat on the common header and have it control both RIBs and modulate both boilers at the same time.

Old school but I don't know of any control that will do what you want although if anyone makes one it would probably be Preferred Utilities Danbury, Ct $$$$. Preferred, if they don't have what you want they can build it.

clammy

I have one simple suggestion and comment . Dis connect the heat timer and run the boilers ,check your heat distribution and and see if any of it jives w the heat timers established heat ,it always seemed to me that by the time the control saw that heat was established and set the heat timer into a run timer that shortly after wards the boilers would start to cycle on pressure mean while most of the indoor spaces where over heating . I know that your situation is different then most I have experienced but all of mine where service related and in disconnecting the heat timer and using temperature averaging thermostat ( one zone system no valves ) that I was able to cut the building owner gas consumption by more then half and stop any short cycling . This may or may not help your situation ,but I some times feel that for buildings using steam for one purpose heating that in many cases a heat timer ( even there top of the line ) has a few design issue that seem to force the boiler to run and short cycle being there is no indoor sensor to help calculate the proper run time and those heat established sensor and run time calculation have never seemed to jive perfectly . As I remember any adjustment done to the controller usually required a visit from heat timer contractor and of course a bill and possibly another adjustment to get it right and bill which is fine and dandy if you are not personally paying the bill but for say a small 15 to 20 unit apartment building the bills for adjustment usually are not made back on savings on fuel or tenants complaining . I find if cold weather shut down is required a simple ranco etd can be used and a few cheap isolation relays . I know that your system is quite large and so must be the building but in respect most faculties that large have lic firemen and operating engineers and usually some type of building automated control system being a heat time would be massively cheaper then a building controls system . Is this facility manafacturing or a office setting.
Peace and good luck clammy
Happy turkey day

EBEBRATT-Ed

Don't know why this didn't attach the first time

Mike_Sheppard

@EBEBRATT-Ed thank you. Lead lag isn’t the problem though. It’s modulation. I need some way to control modulation. But there is no modulation pressuretrol that exists that can work in that low pressure range.

I think I am going to try running the mod motor R and B wires to the existing heat timer and treat them as stages. 

EDIT: Didn’t see your other post before I wrote this! My bad

yeah that’s what I was thinking, using a vapor stat to make them either low or high fire. I’ve done it one time before and it seemed to work. I keep trying to think of how to make it work with this heat timer though since it would be more user friendly for the building engineer.

according to the MPCQ manual it says that it DOES stage the boilers based on that “steam established” setpoint, currently 1.5. I’m not seeing that though. I’ve seen it get as high as 5.5 psi and both boilers still running. They may have minimum run times programmed. If I can straighten the heater timer out I was thinking about running the R/B wire from the existing modulation control over to a relay on the Heat timer to control it as a stage. This would be the cheapest option to try first I think. 

Mike_Sheppard

@clammy it is a condo building. They do have a maintenance guy who is there during the day. Being a condominium, it is the people who live there who make all the decisions, and none of them know anything about steam. I am trying to educate the maintenance team etc.

I haven’t had any complaints yet actually about it being too hot or too cold. The system seems to operate pretty decently. They are chasing a lot of bad traps. No one was doing trap maintenance. Some of the condensate receivers out in the building are master trapped. I showed them how that is not a solution and they need to fix the traps in the building and remove the master traps. They bought a Flir IR gun and are now checking all the thermostatic traps on the radiators and all the F&T traps throughout the basements on the mains. A bit of steam is currently getting back to the feedwater tank, which is probably what killed their old vacuum pumps. Currently the system is not operating with the vacuum pumps because they’re dead. I have a whole new vacuum pump / feedwater tank combo to install this coming summer.

But my issue at the moment is the boiler pressure going too high and no way to control the full modulation of the burners. The heat timer doesn’t have that capability, nor do any of the other controls. Instead of fully modulating them I think I may convert it to two “stages”. It will either be low fire or high fire, nothing in between.

pedmec

I think you had it right at the beginning. I would stage the burners using vaporstats and let the heat timer to control the on/off of the boilers

EBEBRATT-Ed

@Mike_Sheppard

Strange to see a boiler that large without a mod control. If I am not mistaken the boiler runs to the 1.5 psi "heat established" set point and then the boiler starts a timed cycle which is determined by the outdoor air set point and the settings in the heat timer. The warmer it is the shorter the cycle, the colder it is the longer the cycle as I am sure you know

Mike_Sheppard

@EBEBRATT-Ed yea that’s how the heat timer works.

the boilers do have modulation controls. But they’re 0-15 psi. Not nearly accurate enough to control in ounces - 2psi range. There is no vapor stat version of the modulation control.

Heat Timer makes a version that can do the modulation but it cannot perform the steam cycle function. I would have to pair the two heat timers together.

Mike_Sheppard

Turns out Honeywell used to me a 0-16 ounce modulation pressuretrol but they stopped making it. 0-15 psi is the lowest now.

retiredguy

Could you use a steam pressure reducing station which would allow you to utilize a higher boiler pressure and still send the required 1.5psi to the system. In some cases it may also be advantageous to also add a steam "accumulator" (storage tank) to the system.

EBEBRATT-Ed

Automation Direct has pressure transmitters rated for steam that put out a 4-20ma signal. The one I attached may not be the exact model you want but is just for show. Tekmar makes a 005 4-20ma to 135 ohm signal converter to drive the mod motor. This may be workable if the $$$ work out

Steamhead

Another question: Do both boilers need to run to heat the building?

EBEBRATT-Ed

That is why i posted my "lead-lag" drawing. Can't imagine needing both boiler all the time.

But if you let one sit idle it will fill with condensed steam if the valve is left open.

ratio

Honeywell makes an adapter to adapt a Mod4 motor to take a 2-10V/4-20mA signal.

If I couldn't/didn't get better advice from some place like (like …ummm… here), I'd modulate them together to keep one from packing steam into the other. Maybe add a valve to one so that I could stage with one and trim with the other, but I'm a more than a little unsure of how they'd interact.

retiredguy

I like @EBEBRATT-Ed wrote, would prefer a lead-lag system so the lead boiler would provide most if not all the steam necessary to satisfy the building's needs. The lag boiler would supply steam as needed or when the lead boiler fails. To control the lag boiler's water level and keep it from flooding you could install an Armstrong free floating guided lever drain trap or as we called it a "liquid drainer", mounted slightly above the normal water line. When the lag boiler begins to flood, that Armstrong liquid drainer would slowly drain the excess condensate back to the condensate tank. If you really wanted to get technical, you could add "non-return valves" (check valves) at the outlet of each boiler, which is probably overkill for your installation. As far as how to control the boiler's steam pressure I will let that up to guys that are still in the business.

EBEBRATT-Ed

Preferred Utilities makes a lead lag control that will alternate the boilers or you can run either one on lead and the other one on lag or you can build your own with an alternating relay

Mike_Sheppard

The heater timer does lead lag as well, and rotates on time / cycles. I have to keep the time rotation set to 24 hours though because as other have mentioned, the lag boiler will overfill after a couple of days. They should have had high level dump traps on them.

What is there now will handle everything except for the modulation. If only honeywell still made the 0-16 ounce modulation control all problems would be solved.

retiredguy

If you want to install a drain trap on each boiler to drain excess condensate from a boiler to keep it from flooding , look up the Armstrong free floating lever drain trap or as we called it a liquid drainer.