Light Commercial Boiler Piping
Information from the upcoming Hydronics Institute I=B=R courses, now being managed by News parent company Business News Publishing (The News, Sept. 1, 2003), will come from an updated training manual, Residential Hydronic Heating Installation/Design Guide. The 400-page book features more than 300 full-color illustrations, with chapters on boiler selection and placement; steam and hot water components and piping; sizing and auxiliary loads; basic electricity and controls/wiring; and radiant heating.
While the course has a decidedly residential slant, much of the information also applies to light commercial hydronic systems, Coppola said. “You can do a fairly nice-sized building,” with the information provided in the course and book.
He has allowed The News to excerpt the following section on piping for steam heating systems.
‘Smart’ PipingMore than any other heating system, a steam system performs the way it does because the piping is “smart.” Correctly installed, the pipes make the steam and water go when and where they are supposed to go. It may be that, as a hydronic installer, you have more control over performance of a steam system than any other you work with because it’s mostly in the piping.
Some of the best engineering in our business was done in the development of steam heating systems. The installers and designers of these systems didn’t have the automatic controls we have today. And the boiler heat source was hand-fired coal. They accomplished what they did only by installing smart piping.
Probably the single most important improvement in smart piping was the contribution by the Hartford Steam Boiler Insurance and Inspection Co. in 1919 called the Hartford loop. Boiler explosions were common at the turn of the century. One of the primary causes was water backing out of the boiler. (See Figure 1.) Without enough water to cool the boiler properly, boiler surfaces could become red-hot.
When water returned (or a feeder opened), the water flashed to steam instantaneously when it hit the hot surface. The expanding steam tried to find a place to go, but couldn’t. The pressure skyrocketed, tearing the boiler apart.
The Hartford loop prevents water from being pushed out of the boiler. Steam pressure pushes equally (through the equalizer pipe) on the return and supply sides of the boiler. Water can only be pushed as low as the Hartford loop connection, normally located from 2 to 4 inches below the boiler water line (as recommended in the boiler instruction manual).
This is the only “check valve” that really checks. Check valves can accumulate sediment on the seat and leak back, but not the Hartford loop. Water can still be pushed up in the return, but it can’t be pushed out of the boiler.
The Hartford loop is one of the smart piping techniques you control as an installer. Pay close attention to the boiler manufacturer’s piping instructions to be sure you’re using all of the techniques you need for a successful installation. (For information on limit control settings, see related information in the sidebar below.)
Near-Boiler PipingFor a successful steam system installation, you must install the piping near the boiler exactly as shown by the boiler manufacturer. This piping is as important to boiler and system performance as the boiler burner and controls.
You can’t just install the piping “the way the old boiler was installed” when doing a boiler replacement. Too many things are different today. Boilers are smaller for higher efficiency, and they behave very differently from the boiler originally installed with the old steam systems. And the needs of a boiler vary with boiler design.
If the boiler manual shows multiple risers, you have to install multiple risers as shown. And the steam supply piping cannot be taken from between the risers — it has to be taken between the equalizer and the last riser. (See Figure 2.)
Even when properly cleaned and skimmed, the boiler has to deal with dirty, usually contaminated, water. Steam leaving the boiler carries water droplets with it because of the turbulence at the water’s surface. As a comparison for what happens inside a boiler, contrast how a pan of water boiling on the stove behaves. It could steam indefinitely until the pot runs dry, unless you drop in some macaroni. The starch in the macaroni dissolves in the water. Steam bubbles try to break at the surface, but surface tension causes liquid to form around the bubbles instead. The surface begins to foam higher and higher with these starchy bubbles until the pan boils over.
This behavior happens regularly inside a steam boiler because of sediment and impurities carried over from the system piping. In a steam boiler, impurities or chemical additives cause carryover and foaming.
Liquid SeparationBoiler manufacturers provide part of the solution to this problem of liquid carryover in the steam by showing you how to install piping to remove this liquid.
Locate the steam system supply pipe between the boiler equalizer and the last riser as shown in Figure 2. The piping becomes a mechanical separator. The relatively heavy water drops can’t turn as easily as the steam, so they keep moving ahead when the steam turns up into the supply piping, dropping into the equalizer.
If you ignore the boiler manual and pipe the supply from the center, you cause what is shown in the upper right of Figure 2. Water accumulates at the steam supply take-off until it begins to carry over into the system.
Result: water hammer, trap and vent damage, and system flooding because water leaves the boiler too fast, making the system makeup feeder think additional water is needed.
Multiple RisersIf you install fewer risers than the boiler manual requires, you can cause what is shown in Figure 2, lower right. For the steam to move toward the outlet pipe inside the boiler, the pressure where the steam starts has to be higher than where it is going. It wouldn’t move otherwise.
If the boiler needs two risers and you install one, the steam flow inside the boiler is twice what it is supposed to be. If you double the flow, you quadruple the pressure drop.
The water inside the boiler is just like a manometer; apply more pressure on one side than the other and the water will slope as shown. This will cause the right end of the boiler to overheat and would probably cause a cast iron section to crack.
Though boiler piping may sometimes seem complicated, it isn’t optional. The boiler and the system may not work properly if you don’t follow the boiler manufacturer’s guidelines.
The I=B=R classes will be offered in several locations beginning in November. For more information, visit www.bnp.com/conferences or call Keri Wrobel at 248-244-6463.
Sidebar: Limit Control SettingsCut-in pressure: the pressure at which the limit control will close, allowing the boiler to fire. Set the cut-in pressure at about 1/2 psig for gravity-return systems; about 1 psig for pumped-return systems (unless the system piping was designed for a pressure drop higher than 1/2 psig). The rule of thumb is to set the cut-in pressure no less than two times’ the system pressure drop.
Cut-out pressure: the pressure at which the limit control will open, shutting the boiler down. Set the cut-out pressure at 1 psig for gravity-return or 2 psig for pumped-return systems using radiators or convectors. If the system contains a heat exchanger or unit heater requiring more than 1 psig of steam inside, add the required pressure at the heating unit to the cut-in pressure to find the cut-out pressure.
Differential: the difference between the cut-in and cut-out settings of a limit control.
Keep The Pressure Low
Always set the boiler pressure limit control as low as possible. Never turn up the pressure setting to solve a heat distribution problem. Pressure probably isn’t the reason for the heating problem and higher pressure will probably make the problem worse.
The piping in most residential steam heating systems is designed for a pressure drop of less than 1/2 psig, and the radiators will deliver rated capacity of 240 Btuh per square foot equivalent direct radiation (EDR) with only 1 psig of steam inside.
If you set the pressure any higher than 2 psig, you can create problems with operating components, particularly on gravity-return systems.
Publication date: 09/15/2003