Water hammer in steam systems: cause and effect
Heat transfer systems in almost any form %%MDASSML%% hot water, high temperature oil and steam %%MDASSML%% require careful design and operation because of the temperatures and pressures involved. Generally speaking, steam systems are at their safest when condensate is efficiently removed. Proper steam system design, installation, operation and maintenance significantly decrease the potential for destructive events, such as those caused by water hammer.
What is water hammer?
Water hammer occurs when water, accelerated by steam pressure or a low-pressure void, is suddenly stopped by impact on a valve or fitting, such as bend or tee, or on a pipe surface. Water velocities can be much higher than the normal steam velocity in the pipe, especially when the water hammer is occurring at startup. When these velocities are destroyed by impact, the kinetic energy in the water is converted into pressure energy, and a pressure shock is applied to the obstruction.
In mild cases, there is noise and perhaps movement of the pipe. More severe cases lead to fracture of the pipe or fittings with almost explosive effect and consequent escape of live steam at the fracture. Fracturing of pipes or steam system components can propel fragments that can cause injury or loss of life.
There are two types of water hammer: a steam-flow-driven water hammer and condensate-induced water hammer. A steam-flow-driven water hammer is an impact event, where a slug of rapidly moving water strikes a stationary object. The exchange of momentum creates a pressure of perhaps a few hundred psi in the impact area.
A condensate-induced water hammer is the more powerful of the two types. It is a rapid condensation event that occurs when a steam pocket, being totally surrounded by cooler condensate, collapses into a liquid state. Depending on the pressures and temperatures involved, the reduction in volume may be by a factor of several hundred to well over a thousand, and the resulting low-pressure void allows the pressurized surrounding condensate to rush in, resulting in a tremendous collision. This in turn generates a severe over-pressurization that can easily exceed 1,000 psi. Gaskets, fittings and valves %%MDASSML%% virtually any piping component %%MDASSML%% are susceptible to failure, often with tragic consequences.
Common places to look for both types of water hammer are steam mains, steam tracing lines and air heating coils.
Causes of water hammer
Condensate buildup is common to both types of water hammer. It can be caused by boiler carry-over depositing large amounts of boiler water into the steam main, overwhelming the steam traps. Or backflow from the condensate main can be driven by deaerator pressure and perhaps flash steam through malfunctioning steam traps or check valves. Even reduced trap capacity caused by low steam main pressure conditions can result in backflow from condensate mains.
An element necessary to steam-driven water hammer is steam flow, usually from some sort of nearby steam load producing the force that drives the slug.
For condensate-induced water hammer, necessary elements are:
steam condensation, during which a trapped pocket of steam condenses in pooled condensate
a pressure drop (perhaps combined with steam process control valves opening), which can lead to a vacuum being created
Reducing the risks
Operators can reduce their risk of water hammer by preventing or resolving steam system design issues.
1 . Drainage: Avoid water hammer completely by taking steps to ensure that water (condensate) is drained away before it accumulates in sufficient quantity to be picked up by the steam. Provide proper drainage; do not simply deal with it by installing components with high pressure ratings or capacities. Components with generous “safety factors” do not necessarily ensure safe and effective steam main drainage.
2 . Steam quality: Improve steam quality, keeping steam as dry as possible at all times. Install steam-conditioning stations upstream of meters and any other critical steam system components.
3 . Steam velocities: Do not allow steam velocities to become excessive as a result of system modifications. The higher the velocity, the higher the force of impact during a steam flow-driven event.
4 . Boiler and steam supply: In larger systems, consider installing an automatic valve in the steam supply line, arranged so that the valve stays closed until a reasonable pressure is attained in the boiler. The valve can then be set to gradually open, allowing flow, temperature and pressure in the distribution system to reach equilibrium slowly. Install a backpressure control valve on the steam main to prevent the pressure within the boiler itself from being drawn down due to some sort of upset condition.
5 . Steam traps: Make sure the steam traps used are of correct type and capacity. Type can depend on the startup methods used. If operational procedures change, different types of steam traps may be needed. If in doubt, call in a steam system expert. Check steam traps regularly and maintain them properly. Never fall below a minimum pressure differential across a steam trap. Always pipe steam main isolation valves with a steam trap to allow drainage of condensate that may form while the valve is closed. Design target unit piping to include bypass systems that allow gradual heating and pressurization on startup.
6 . Piping: Correct any occurrences of pipe sagging and missing, wet or damaged insulation that could cause condensate accumulation and exceed steam traps’ capacities.
7 . Air heating coils: These units must accommodate both condensate removal and air venting to prevent water hammer. In “horizontal” coils, the tubes should not be horizontal but should have a slight fall from inlet to outlet so that condensate does not collect in pools but drains naturally. Steam inlets to “horizontal” headers may be at one end or at mid length, but with vertical headers the steam inlet is preferably near the top.
Coils with a center inlet connection make it more difficult to ensure that air is pushed from the top tubes; the steam tends to short-circuit past these tubes to the condensate header. Automatic air venting of the top condensate header of these coils is essential. With other layouts, an assessment must be made of the most likely part of the unit in which air and non-condensable gases will collect. If this is at the natural condensate drain point, then the trap must have superior air venting capability. A float-thermostatic type is the first choice. A vacuum breaker should be installed in the steam supply pipe between the temperature control valve and the coil inlet.
Avoiding water hammer in practice
Water hammer can also be avoided by preventing or resolving certain operational issues:
1 . Dangerous mix: High-pressure steam in contact with sub-cooled condensate is an unstable and potentially explosive mixture.
2 . Cooled condensate: Do not allow steam to be admitted into a line that is suspected of containing sub-cooled condensate.
3 . Boilers: Make sure that the boilers operate correctly under all load conditions, without foaming or carryover.
4 . Steam pressure: Be careful when the pressure in the steam main is low or zero. Because of the lift after the drip traps, flooding will occur even with some pressure in the steam main. Be especially careful during startup, when boiler goes down and when process loads overwhelm steam capacity.
5 . Startup and shutdown: These are critical operations. Never allow a steam system to shut down or restart without operator involvement. In larger systems, use a supervised startup procedure. Open manual drain valves, such as the ones on the bottom of the drip pockets or the blowdown valves off the strainers until the steam main has enough pressure for the steam traps to take over. In any but the smallest plants, the flow of steam from the boiler into the cold pipes at startup, while the boiler pressure is still only a few psi, will lead to excessive carryover of boiler water with the steam. Such carryover can be enough to overload separators in the steam takeoff, where these are fitted.
6 . Malfunctioning steam traps: If a trap is found to be failed closed, temporarily set the strainer blowdown valve partially opened to allow condensate to bleed. If a trap is found to be failed open, it should not be valved shut—this would allow condensate to build up in the steam main. Be sure to implement a program of regular steam trap testing to maintain the highest level of trap performance.
7 . Air heating coil operation: Stalling is the most common cause of heating coil problems. Stalling occurs when pressure within the steam space falls under part load conditions. If pressure falls to the level that condensate flow to the traps ceases, the system “stalls.” As condensate backs up into the coil, waterlogging problems of hammering, temperature stratification, corrosion and freeze-up begin. A waterlogged coil should gradually drain itself freely to a downstream trap and, from the trap, by gravity to a vented receiver and return pump. If it doesn’t, suspect a missing or malfunctioning vacuum break on the coil. A better strategy is to use an automatic pump trap, the newest technology combining the benefits of a float trap with those of a pressure powered pump to ensure effective drainage of condensate from the steam space, no matter the pressure.
Water hammer event forensics
A West Coast petrochemicals plant experienced a steam system incident that resulted in an instantaneous fracture and total separation of a 10-inch gate valve. The back half of the valve, along with the attached blind flange, was launched from the pipework and landed approximately 50 feet away on the asphalt driveway.
The total plant-wide steam load was in excess of 200,000 lb/hr. The valve was located at the end of a 150-psi steam main located on the roof of a process building. A float and thermostatic steam trap was located close to the 10-inch gate valve. Condensate from the drip traps in most cases had a few feet of lift to the return main. The boiler pressure reportedly dropped for unknown reasons. The pressure seemingly decreased to as low as 30 to 35 psi, then was being increased slowly. It was at this point in time that the event occurred.
The deaerator pressure was about 5 psi at the time of this survey. The float and thermostatic steam trap located close to the 10-inch gate valve was found to be damaged; whether the damage actually occurred during the same event that ruptured the gate valve is impossible to determine. However, the combined evidence of the damaged gate valve and the steam trap float point to water hammer as the most likely culprit. The questions would then be: a) what type of water hammer, and b) why did it occur.
As a point of information, 10-inch schedule 40 pipe is capable of about 70,000 lb/hr without incurring excessive velocities and pressure drop; 12-inch, about 100,000. Higher velocities will increase capacities by about 50% and pressure drops by more than 100%.
If the event were caused by steam-driven water hammer, then some sort of steam load should have been nearby. This was not the case. Additionally, the magnitude of the damage incurred would point to condensate-induced water hammer. Two factors would be necessary: a condensation-induced vacuum and condensate. The drop in the steam system pressure may have some bearing here, leading to a vacuum being created. Under the low-pressure conditions, the steam traps’ capacity could have decreased to the point where no condensate would drain from the main. Low pressure together with condensate system backpressure could have combined to cause condensate flow back into the steam main.
The case clearly illustrates that it is often impossible to determine with any certainty the actual cause or causes of damage in the wake of a water hammer event. If you suspect an impending water hammer condition in your steam system, you should undertake a careful review of the remediation suggestions presented. If any doubt remains, engage a qualified steam system engineer to conduct an audit of design and operational factors.