Good Piping Practice Prevents Water Hammer in Steam Systems
One of the most common complaints against steam heat is that a system sometimes develops a hammer-like noise commonly referred to as water hammer. It can be very annoying. However, it may indicate a condition which could produce serious consequences including damaged vents, traps, regulators and piping.
There are two types of water hammer that can occur in steam systems.One type is usually caused by the accumulation of condensate (water) trapped in a portion of horizontal steam piping.
The velocity of the steam flowing over the condensate causes ripples in the water. Turbulence builds up until the water forms a solid mass, or slug, filling the pipe. This slug of condensate can travel at the speed of the steam and will strike the first elbow in its path with a force comparable to a hammer blow. In fact, the force can be great enough to break the back of the elbow. Steam flowing in a system at 10,000 feet per minute is traveling more than 100 miles per hour. The slug of condensate is carried along by the steam flow.
The second type of water hammer is actually cavitation. This is caused by a steam bubble forming or being pushed into a pipe completely filled with water. As the trapped steam bubble looses its latent heat, the bubble implodes, the wall of water comes back together and the force created can be severe. This condition can crush float balls and destroy thermostatic elements in steam traps. Cavitation is the type of water hammer that usually occurs in wet return lines or pump discharge piping.
A properly piped steam system should not produce water hammer of either type.
Water Hammer in Steam Lines
Water hammer in steam lines is normally caused by the accumulation of condensate.
Important installation details to prevent water hammer in steam lines include the following:
Water Hammer in Condensate Return Lines
In most installations, water hammer in condensate return lines is caused by steam pockets forming and imploding. Frequently, the cause is a rise in the discharge line from a trap or a high pressure trap discharging into a low temperature wet return line.
A lift in the return line after the trap will cause water hammer because the temperature of the condensate leaving the trap exceeds 212°F. The high temperature condensate flashes, causing steam bubbles to form. As these steam bubbles are pushed into colder condensate in the return piping, they implode and cause water hammer. The water hammer will normally be worse during start up due to the cold condensate lying in the return piping. As the temperature of the return line increases above 212°F the water hammer often stops. Many industrial applications install lifts to avoid installing additional condensate return systems. When installing a lift, the most commonly used trap is an Inverted Bucket Trap since the open bucket design tolerates moderate water hammer check valve helps isolate the trap from the water hammer forces and prevents back flow of condensate when the steam supply is secured.
When a trap discharges into a wet return line, flashing will occur. Again, these steam bubbles implode causing water hammer. This condition is often found where a high pressure drip trap is connected into a pumped return line with lower temperature condensate. Older versions of the ASHRAE guide showed the use of a diffuser pipe to break up the high temperature condensate to reduce the size of steam bubbles that occur. The guide showed welding a pipe tangentially in the return line and drilling 1/8 inch holes at least 1 inch apart.Other methods include using a heat exchanger to blend the two temperatures or the use of fin tube radiation to cool the trap discharge.
The most common method used is to install a flash tank on the drip trap discharge allowing the condensate to flash to 212°F and then pumping the cooled condensate into the common return line.
Important installation details to prevent this type of water hammer are listed below.
1) Whenever possible, use gravity return lines. Properly sized return lines allow condensate to flow in the bottom portion of the pipe and flash steam to flow in the top portion of the pipe. The top portion also allows efficient air venting during start up of the system.
2) Water hammer can occur in pumped discharge lines. A condensate unit is pumping condensate near saturation temperature to an overhead horizontal run and then drops down into a vented boiler feed tank. A negative pressure develops in the horizontal pipe due to the piping drop into the vented receiver. When the pressure falls below saturation temperature, water hammer can occur. A 12 foot vertical drop can allow 190°F condensate to flash and cause water hammer. This condition can be remedied by either creating a back pressure at the low point or by installing a swing check valve open to atmosphere in the horizontal pipe. The swing check will open, allowing air to enter and the vertical water column to drain away.
This condition can also occur in the boiler feed pump discharge line from a deaerator or pre-heat unit. In many installations, the discharge lines run overhead, a check valve or regulator valve is installed near the boiler, and a check valve is installed at the pump discharge. If the check valve at the pump discharge does not hold tight, condensate drains back to the boiler feed unit, allowing the condensate in the discharge to flash. A steam pocket forms at the high point. The result is water hammer when the pump starts. This can be corrected by replacing the check valve.
Heat Exchanger Installation to Prevent Water Hammer
The steam trap must be capable of completely draining the condensate from the heat exchanger shell under all operating conditions. On a heat exchanger using a modulating temperature regulator to heat fluids under 212°F, the steam pressure in the shell can be 0 psig. To ensure condensate drainage, the steam trap must be outlet tapping and it must drain by gravity into a vented condensate return unit. When possible, the trap should be located 15 inches below the heat exchanger outlet. The 15 inches of static head to the trap inlet will provide 1/2 psig static inlet pressure to the trap when the shell steam pressure is at 0 psig. The trap should be sized based on 1/2 psig differential pressure. A safety factor of 1.5 times the calculated full load capacity should be used to handle unusual start up loads. A float and thermostatic trap is normally the best selection for a heat exchanger. The thermostatic element quickly vents the air from the heat exchanger shell. The modulating float element provides continuous condensate drainage equal to the system condensing rate. (See Figure 1.)
Failure to provide complete condensate drainage will lead to poor temperature control and possible water hammer.
Any lift in the condensate return piping after the trap discharge requires a positive pressure to develop in the heat exchanger shell to provide condensate drainage. For this to occur, condensate needs to back up in the heat exchanger shell until enough tube surface is covered by condensate to build a positive steam pressure. However, when the positive steam pressure develops to move the condensate through the steam trap and up the vertical return line, over temperature can occur from the steam remaining in the shell. The resulting condition will show a wide range of outlet fluid temperature from the heat exchanger tube side.
A lift or back pressure in the steam trap return piping can flood the heat exchanger shell and cause severe water hammer as steam enters the flooded shell. The resulting water hammer can damage the steam trap, the steam regulating valve, and the heat exchanger tubes. It can also cause the heat exchanger and trap gaskets to blow out.
The return line from the trap discharge should be pitched into a vented condensate return unit. (See Figure 2.)
Most steam to water heat exchangers provide a tapping in the shell to allow installation of a vacuum breaker. The vacuum breaker allows air to enter the shell if a vacuum is induced. Failure to install a vacuum breaker will allow the heat exchanger shell to operate at a negative pressure which may cause condensate to hold up in the shell causing water hammer.
Steam systems offer a lot of advantages for distributing heat in large facilities. When the systems are properly installed they provide years of quiet, trouble-free operation.
This is an abridged version of an ITT Fluid Handling article that appeared in the August 1998 issue of Engineered Systems magazine.
Reprinted from TechTalk September 1998
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