2000 Hook-up Book - Spirax Sarco

Parallel Operation
In steam systems where load
demands fluctuate through a wide
range, multiple pressure control
valves with combined capacities
meeting the maximum load perform
better than a single, large
valve. Maintenance needs, downtime
and overall lifetime cost can
all be minimized with this arrangement,
Fig. 38 (page 20).
Any reducing valve must be
capable of both meeting its maximum
load and also modulating
down towards zero loads when
required. The amount of load
turndown which a given valve can
satisfactorily cover is limited, and
while there are no rules which
apply without exception, if the low
load condition represents 10% or
less of the maximum load, two
valves should always be preferred.
Consider a valve which
moves away from the seat by 0.1
inches when a downstream pressure
1 psi below the set pressure
is detected, and which then passes
1,000 pounds per hour of
steam. A rise of 0.1 psi in the
detected pressure then moves
the valve 0.01 inches toward the
Parallel and Series Operation of Reducing Valves
Case in Action: Elimination of Steam Energy Waste
As part of a broad scope strategy to reduce operating
costs throughout the refinery, a plan was established to
eliminate all possible steam waste. The focus of the plan
was piping leaks, steam trap failures and steam pressure
optimization.
Programs having been previously established to
detect/repair steam trap failures and fix piping leaks, particular
emphasis was placed on steam pressure
optimization. Results from a system audit showed that a
considerable amount of non-critical, low temperature tracing
was being done with 190 psi (medium pressure)
steam, an expensive overkill. It appeared that the medium
pressure header had been tapped for numerous small
tracing projects over the years.
Solution
Refinery engineers looked for ways to reduce pressure
to the tracer lines. Being part of a cost-cutting
exercise, it had to be done without spending large sums of
capital money on expensive control valves. The self-con-
seat and reduces the flow by
approximately 100 pph, or 10%.
The same valve might later
be on a light load of 100 pph total
when it will be only 0.01 inches
away from the seat. A similar rise
in the downstream pressure of
0.1 psi would then close the valve
completely and the change in
flow through the valve which was
10% at the high load, is now
100% at low load. The figures
chosen are arbitrary, but the principle
remains true that instability
or “hunting” is much more likely
on a valve asked to cope with a
high turndown in load.
A single valve, when used in
this way, tends to open and close,
or at least move further open and
further closed, on light loads. This
action leads to wear on both the
seating and guiding surfaces and
reduces the life of the
diaphragms which operate the
valve. The situation is worsened
with those valves which use pistons
sliding within cylinders to
position the valve head. Friction
and sticking between the sliding
surfaces mean that the valve
head can only be moved in a
tained cast steel pressure regulators and bronze reducing
valves were chosen for the job. In 1-1/2 years, approximately
40 pressure regulators and hundreds of bronze
reducing valves have been installed at a cost of $250K.
Annualized steam energy savings are $1.2M/year. More
specifically, in the Blending and Shipping Division,
$62,640 was saved during the winter of 1995, compared to
the same period in 1994.
Benefits
• Low installed cost. The Spirax Sarco regulators and
bronze reducing valves are completely self-contained,
requiring no auxiliary controllers, positioners, converters,
etc.
• Energy savings worth an estimated $1.2M/year.
• The utilities supervisor who worked closely with Spirax
Sarco and drove the project through to successful completion
received company wide recognition and a
promotion in grade.
series of discreet steps.
Especially at light loads, such
movements are likely to result in
changes in flow rate which are
grossly in excess of the load
changes which initiate them.
Load turndown ratios with pistonoperated
valves are almost
inevitably smaller than where
diaphragm-operated valves are
chosen.
Pressure Settings
for Parallel Valves
Automatic selection of the valve
or valves needed to meet given
load conditions is readily
achieved by setting the valves to
control at pressures separated by
one or two psi. At full load, or
loads not too much below full
load, both valves are in use. As
the load is reduced, the controlled
pressure begins to increase and
the valve set at the lower pressure
modulates toward the closed
position. When the load can be
supplied completely by the valve
set at the higher pressure, the
other valve closes and with any
further load reduction, the valve
still in use modulates through its
own proportional band.
21
SYSTEM DESIGN