Maintenance Technology April 2017 - 34

OIL
&
GAS
processing
Case Study

Surge Vessels Address
Hydraulic Shock
Properly implemented surge vessels can optimize
pump/piping-system performance and address hydraulic shock.

THE ISSUE
Under steady-state conditions, a plant's pumping system will tend to
operate near the nominal working pressure unless there is change
of ﬂow velocity. This change is deﬁned as hydraulic shock and
immediate mitigation efforts are needed to prevent damage from
occurring.
This ﬂuid acceleration or deceleration can be attributed to several likely causes, with the most common being from either "pump
trip," or sudden valve closure. A pump trip, generated by sudden
loss of power to the pump station or by a pump stop without warning, can drop the working pressures near the pump's discharge side
to negative levels and cause possible vapor-pocket collapse.
The sudden valve closure from electrical, hydraulic, or
mechanical failure, or from human action, can result in a dramatic
increase in pressure at the inlet side of the closed valve. That pressure increase is experienced as high-velocity (potentially exceeding
4,000 ft./sec.) transient pressure waves that will oscillate throughout
the piping network unless the transient wave energy can be suppressed.
Pipes that shake violently, even occasionally with restrained
piping, and with loud banging noises are the ones typically experiencing hydraulic shock. Pumps and motors are also likely to be
damaged concurrently as the transient-pressure energy waves
travel back through the pump until the check valve slams shut.
Weak points in the piping network, such as ﬂange connections
and pipe elbows, tend to bear the brunt of the pressure wave's

34 |

RELIABLE PUMPING SUPPLEMENT

FIG. 1: NEGATIVE-PRESSURE
TRANSIENT WAVE
Pressure (psi)

amage to pumps and piping systems from hydraulic shock,
also known as water hammer, can often result in catastrophic failure, along with expensive repair and downtime.
In the world of petrochemical processes, hazardous conditions
resulting from pump damage or line breaks can also bring about
signiﬁcant liability concerns, along with very negative publicity. With
many plants and facilities currently in operation without protection
against hydraulic shock, what can be done from a maintenance,
repair, and operations (MRO) standpoint to avoid this inevitable
problem?

Time (sec.)

Graph shows a transient negative-pressure wave on a
pump's discharge side that occurs when power is lost to
a booster pump. Green shows booster-pump pressure
and red shows high-pressure-pump pressure.

FIG. 2: PRESSURE VARIATION
WITHOUT A SURGE VESSEL
Pressure (psi)

FRANK KNOWLES SMITH III
AND STEVE MUNGARI,
BLACOH FLUID
CONTROLS INC.

Time (sec.)

Fig. 2 shows pressure variation in a system that is not
equipped with a surge vessel. Green is the booster-pump
pressure and red is high-pressure-pump pressure.

damaging effect and are often the ﬁrst to break.
In a single-pump system, several transient-mitigation options
are available to address the transient wave's effects. Some of
the most popular are surge vessels, air-release/vacuum valves,
pressure-relief valves, surge-anticipator valves, and vacuum breakers. Even with an existing facility or pipeline, space is often readily
available to accommodate which speciﬁc pieces of mitigation equip-

APRIL 2017

Table of Contents for the Digital Edition of Maintenance Technology April 2017