A Good Time to Check a Vacuum Steam System
Why do we work on heating systems during the winter? Because we’re working on air-conditioning systems during the summer! Makes sense in a last-minute, panicky way, but it’s not the best way to deal with either system, especially when it comes to vacuum steam systems. With these systems, the radiator- and end-of-main traps are often the source of all the problems (they’ve failed in the open position), and fixing these during the warmer months is the way to go. If you try to fix them during the winter you can’t do them all at once, and the bad ones will kill the good ones as quickly as you can install the good ones. Steam in the return lines can be vicious.
And while you’re checking those traps, poke around and ask the folks in the building if the place heats unevenly. If they say that it does it may be because the vacuum pump isn't producing the right vacuum. In a heating system, a vacuum pump usually produces vacuum in a range of three-to-eight inches of mercury (Dead Men sized for an average of 5-1/2” hg vacuum return on new systems). If the pump has to lift condensate from a low point, either through lift fittings or an accumulator tank, the pump has to be able to produce an additional inch of mercury vacuum for every foot of vertical lift. This is costly nowadays, and I’d do my best to avoid it if I could work around it with a re-pipe.
If the condensate is too hot (because of failed steam traps, or if you’re running too high a supply pressure), or if the system has leaks, the vacuum pump won't produce the vacuum you need to move steam quickly through the system, so always check the temperature of the returning condensate. Ideally, it shouldn't be hotter than 180 degrees.
To check the vacuum pump's operation, close the inlet valve to the vacuum pump's receiver. If the pump starts to pull vacuum with the valve closed, but you’re not able to get the vacuum with the valves open, look for leaks in the system. I know that’s easier said than done, but you need to do it. And also check to see if someone had added vacuum breakers or air vents anywhere in the system. Vacuum makers and vacuum breakers don’t get along well. It’s like having a humidifier and a dehumidifier in the same room.
The system might also have a condensate transfer pump at a low point. We use these pumps instead of less reliable accumulator tanks and lift fittings. The vent line on the transfer pump's receiver has to connect into a dry return line, downstream of the steam traps. If someone left the vent line open to atmosphere, the vacuum pump won't produce vacuum.
Make sure the vacuum pump has enough hurling water. This is the water in the upper chamber of the vacuum pump. Without hurling water, the vacuum pump can't produce vacuum. You'll find a float switch in the upper chamber. The float switch operates the solenoid valve that admits the hurling water. Check both the switch and the solenoid valve.
Does the boiler flood? If so, look to see if the system has a vacuum pump and a boiler feed pump. As you probably know, when you replace an old boiler with a new boiler, you sometimes need to use a boiler-feed pump. The boiler-feed pump acts as a reservoir to hold the water that used to be in the old boiler, but isn't in the new boiler. The boiler-feed pump's job is to keep the boiler from flooding, and from shutting down on low water.
If the Dead Men sized the system's pipes, valves, fittings and steam traps for a pressure-to-vacuum differential, you have to use a vacuum pump. Without the vacuum pump, steam won't flow quickly from the boiler to the radiators. You'll have to use higher-than-normal pressure and the system will heat unevenly.
When you have both a vacuum pump and a boiler-feed pump, the boiler feed pump will be closest to the boiler. When the vacuum pump starts, it will pull vacuum right back to the boiler’s waterline. As the vacuum forms within the boiler and the system piping, atmospheric pressure inside the boiler-feed pump will push water from the tank’s reservoir and into the boiler, causing the boiler to flood. It’s a common problem and one that can have you scratching your head.
To solve the problem, install a motorized valve on the boiler-feed pump's discharge line. Then use the boiler-mounted pump controller to operate the motorized valve, and have the valve start the boiler-feed pump through its end-switch. A simple, and relatively low-cost solution.
Now how about if the condensate returns too slowly from the system? What’s up with that? Well, let’s take a look. A vacuum pump operates like a condensate pump. When its receiver fills, the vacuum pump returns the water to the boiler. The vacuum pump has no way of knowing whether the boiler needs this water or not (which is why you’ll often see that boiler-feed pump in addition to the vacuum pump).
If some steam traps have failed, the condensate will usually return slowing because there will be less pressure differential between the mains and the returns. The automatic water feeder will add water to the system to keep the boiler operating. When the condensate finally does return to the vacuum pump, the pump will add this water to the boiler, and the boiler will flood.
If the steam traps have failed in the opened position, steam will pass into the return lines and heat the condensate. The vacuum pump can't pump water when it's that hot. The pump will cavitate, and little or no water flows back to the boiler. If there's no automatic water feeder on the job, the boiler will go off on low water. Steam traps have to be working on these systems, and the warmer months are when you should be checking them.
If you’re getting water hammer in the returns, it’s usually because the returns don’t have the proper pitch. The vacuum pump's main job is to remove air from the system, not to return condensate to the boiler. To put it another way, it’s not supposed to suck water, just air. As in a pressure system, water should be returning to the boiler solely by gravity. If the pitch on the returns isn't continuously downhill, condensate will accumulate in the returns. Depending on the system pressure, flash steam from the steam traps can enter the returns when the traps open (or if the traps have failed). The steam will create water hammer when it meets the accumulated condensate in the returns and you can be a hero if you check those traps and that pitch.
Wander around; be nosey, and ask lots of questions. And do it when it’s warm outside.