Pipeline Pressure
Relief Valves
The best relief valve is the one that operates most simply, and most applications
can be satisfied with spring-loaded or nitrogen-loaded relief valves, say J.R.
Chester.
This article discusses the application,
use and requirements
of pressure relief
valves for pipeline overpressure protection.
In addition, overall design
of the relief system is discussed, including
fail safe protection, microprocessor-
based step control, the
need for fast action and a brief explanation
of hydraulic transient
pressure waves.
Over pressurisation of a pipeline is
commonly caused by sudden changes
in liquid velocity, such as when a pump
starts or stops or a valve opens or closes.
When a pressure rise occurs, above
normal operating pressure, it is very
important to analyse the rate of the
pressure rise to determine the proper
size and type of valve required.
Relief valves of many types and
sizes have been developed to relieve
excess pressure, but the correct
valve(s) must be selected to properly
and continually protect the pipeline.
Common Causes of Pipeline
Overpressure
A pipeline engineer assigned the task
of designing protection for a liquid
pipeline must have a basic knowledge
of pipeline hydraulics. The basic protection
can be classified in three areas,
as discussed here.
Thermal or Product Expansion
During Line Static Conditions
These are normally classed with spring-loaded relief valves in small
sizes used to protect the pipeline and
associated equipment in a static
state due to thermal expansion of the
product. Thermal relief valve applications
are not discussed in this article.
Line Blockage
This is the most serious pipeline
problem, and every effort is made by
valve interlocking and operating procedures
to operate without line
blockage. Fig. 1 illustrates line blockage
and shows that the pressure at
“B” will exceed the initial pumping
head with no relief as the product
packs into the closed line segment.
At time 200, the flow reverse on the
“A” side of “B” and the reverse flow
begin dropping the pressure. The
pressure at “B” is composed of surge
pressure and packing pressure.
Surge pressure is created by valve
closure. Packing pressure occurs after
valve closure. Line packing is 2
to 3 times the surge pressure and
thus is the most serious. The surge
pressure is created during the last
20% of valve closure and may be as
170 psi/second. Total surge pressure
will be about 30+ times the velocity
in feet/second. Packing varies with
line diameter, wall thickness and
length. In most pipelines, the problem
is line packing. Large diameter,
high flow pipelines must be designed
for surge protection as well as line
packing.
A recording showing pressure
versus time and valve travel is
shown in Fig. 2. This shows a 36-inch
valve in a product line closing in 10.3
seconds. This caused a 160-psi pressure
in the last second of valve closure.
The pressure surge created by
valve closure was 320 psi. The packing
continued to rise for approximately
110 seconds and reached 660
psi before a negative wave from the
upstream station caused a pressure
drop. A large relief valve, which must
be fast enough to handle the surge
cont....
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