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Article # 0063
Determination of the
Cause of Water Hammer in Two Steam Pressure Letdown Stations
By Richard L. Jones PE
Steam Pressure Letdown Stations are common in large industrial
facilities such as refineries and chemical plants and especially in
cogeneration plants that are providing them with steam. Often
the steam being reduced in pressure and temperature starts out as very
high pressure superheated steam. The stations are comprised
of a control valve for pressure reduction and a separate desuperheater
for temperature reduction but many have more recently evolved to steam
conditioning valves where both the pressure reduction and temperature
reduction are accomplished within a single valve body. In
either case the application has the potential for experiencing
significant water hammer and requires proper design and maintenance to
ensure that water accumulation and hammer is
In 2013 a Houston area cogeneration plant experienced a major water
hammer event in one of two identical and parallel steam letdown
stations. The main line was moved several feet from the shock
destroying concrete pipe supports and breaking the smaller drain
piping. Each station includes a steam
conditioning valve (SCV), spray water control valve (SWV), safety
valve, and drip leg with strainer and steam trap. Process conditions
are 850 psig and 750 degrees F inlet and 175 psig and 400 degrees F
outlet. Installation was in 2004.
The system layout has two parallel and interconnected stations each
with a 10” inlet vertically down into the angle-bodied SCV with a 20”
outlet horizontal in a straight run of 20 ft. The outlet line
then makes a 90 degree horizontal turn and extends another 20 ft. at
which point the line turns vertically up and ultimately ties into the
plant’s 175 psig steam header. An isolation valve is located
in the vertical up section of the discharge line. The SWV is
installed approximately 10 ft. upstream of its connection to the
SCV. The SCV has a body drain and steam trap at the low point
of the valve which is also the low point of the high pressure steam
inlet line. The low pressure drip leg is located immediately
prior to the elbow where the line turns vertical upward. The line from
the SCV to the drip leg is sloped toward the drip leg to ensure any
condensate or spray water fallout drains into the drip leg.
There are several ways water could become present in the line in
sufficient quantity that could result in water hammer. The source could
be either upstream or downstream of the SCV. The upstream
option is that condensate can form in the dead-leg section of the high
pressure superheated line upstream of the SCV during periods when the
SCV is closed and there is a failed-closed, plugged, valved-out or
missing steam trap in the body drain of the valve. When water
hammer occurs due to water present at the inlet of the valve at
start-up there is usually valve damage that includes the diffuser (when
one is installed) being blown out of the valve outlet into the
downstream piping. The downstream option can occur from one
or more of multiple possibilities: a) Condensate can form in the
dead-leg section of the low pressure line downstream of the
SCV during periods when it is closed and there is a failed-closed,
plugged, valved-out or missing steam trap b) A poorly performing SCV !
or poorly designed installation can result in excess spray
water which falls out of the steam stream without evaporating and in
sufficient volume that the steam trap is unable to handle the load c)
an incorrectly controlled or a leaking or open SWV can allow water to
be injected in the line when there is inadequate or no steam.
The preliminary site inspection began with the SCV which was in the
shop with the outlet accessible to view. There was no
apparent damage to the valve internals including the outlet
diffuser. Had the water hammer originated upstream
of the SCV there would probably have been visual damage. The
diffuser is generally blown out of the valve into the downstream
piping. It was therefore assumed that the water build-up was
after the SCV.
There was concern that the other letdown station could experience the
same type of damage and become unavailable. While still in
operation the insulation was removed and a temperature profile was made
360 degrees around the 20” pipe just upstream of the drip
leg. The results were that there was a 100-110 degree
difference between the upper and lower areas of the pipe indicating
significant accumulation of water in the pipe. Further the
trap was found to be in service (not valved-out) but cold and not
operating and either failed closed or plugged. The trap
bypass was opened and significant water drained from the
line. It was clear that the second station was in danger of
experiencing a serious water hammer. The station was removed
from service until the cause of the excess water was determined and the
system modified to resolve the problem.
The following actions were taken and details assimilated for analysis
and the best possible prediction of the cause of the water hammer:
a) Inspection of the physical system
b) Review of isometric drawings
c) Review of operating trends including past start-ups
d) Calculations using the process conditions to confirm suitability of
e) Discussions with operators and witnesses to the event
The analysis yielded the following results:
a) The system appears to have been designed properly and consistent
with the manufacturer’s general recommendations and good engineering
practice except that the location of the temperature sensor/transmitter
may be a bit close to the SCV.
b) The application is not an easy one as high flow rangeability is
required. This high rangeability is easily handled in the
pressure control but is most difficult for the temperature
control. This is even more challenging considering the small
difference between the set point and the saturation
temperature. Any other issue such as wear on equipment, low
water temperature, etc. only makes the temperature control more
c) Based on the original conditions the percentage of water addition is
fairly high at approximately 15%.
d) A study of trends does not reflect an abnormality in the SWV stroke
and water flow relative to the SCV stroke and steam flow.
Thus there does not appear to be an excessive amount of spray water
being added to the steam.
e) Operators had frequently heard loud hammering noises consistent with
small volumes of water in a steam line being picked up by the fast
moving steam and impacting the pipe at changes in direction.
f) Significant water was seen discharging from the safety valves during
the event. The safety valves are located a few feet upstream
of the elbow where the 20” line turns vertical up.
This indicates significant water in the line being moved at high
velocity and resulting in an effective high pressure at the safety
Based on the analysis, it appeared that the source of the water was not
a serious failure of the steam conditioning equipment with large water
fallout overwhelming the drainage system but was probably due to a
combination of condensate from a period where the steam conditioning
valve was closed for an extended period and/or some spray water
fall-out from the desuperheating process at low flow conditions over an
extended period. However at the reasonably small rates of
water accumulation the installed steam traps should have readily
handled the load. It was therefore reasoned that the entire
issue was the failure of the steam trap to drain the low pressure side
of the SCV.
The steam traps and upstream strainers were cut from the lines and
disassembled in the shop. The strainers located immediately
upstream of the traps were totally plugged thereby eliminating the only
downstream condensate drainage possible in both lines. With
the trap removed from service, even a small amount of spraywater
fallout would eventually create adequate water for such a hammer to
Recommendations were provided to minimize the possibility of such water
hammer events from occurring in the future:
a) Install a level indicator with switch on each drip leg to allow
visual observation of the water level and send an alarm to the control
b) Install suitable trap bypass valves in case an extended blowdown is
required in the future.
c) Install the trap discharge piping to drains such that the discharge
can be visually observed and this used in the determination of trap
performance and estimation of load.
d) Interlock SCV and SWV to ensure the SWV never opens without SCV
e) Regularly inspect the system and review the trends for any sign of
problems or change in operation and performance.
f) Regularly blow down the Y-strainers upstream of the traps.
g) Begin a regular program of steam trap testing. Prioritize
the most critical traps and test them more often.
About the Author
Richard Jones, PE is the President of Richard L. Jones, Inc. and a 1975
Nuclear Engineering graduate of Texas A&M University.
P E Registration Number: 51266
Article # 0063
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