2000 Hook-up Book - Spirax Sarco

SYSTEM DESIGN
22
Parallel and Series Operation of Reducing Valves
This can be clarified by an
example. Suppose that a maximum
load of 5,000 lb/h at 30 psi
can be supplied through one
valve capable of passing 4,000
lb/h and a parallel valve capable
of 1,300 lb/h. One valve is set at
29 psi and the other at 31 psi. If
the smaller valve is the one set at
31 psi, this valve is used to meet
loads from zero up to 1,300 lb/h
with a controlled pressure at
approximately 31 psi. At greater
loads, the controlled pressure
drops to 29 psi and the larger
valve opens, until eventually it is
passing 3,700 lb/h to add to the
1,300 lb/h coming through the
smaller valve for a total of 5,000
lb/h.
There may be applications
where the load does not normally
fall below the minimum capacity
of the larger valve. It would then
be quite normal to set the 4,000
lb/h valve at 31 psi and to supplement
the flow through the 1,300
lb/h valve at 29 psi in those few
occasions when the extra capacity
was required.
Sometimes the split between
the loads is effectively unknown.
It is usual then to simply select
valves with capacities of 1/3 and
2/3 of the maximum with the
smaller valve at the slightly higher
pressure and the larger one at
the slightly lower pressure.
Two-Stage
or Series Operation
Where the total reduction in pressure
is through a ratio of more than
10 to 1, consideration should be
given to using two valves in series,
Fig. 39 (page 20). Much will depend
on the valves being used, on the
total pressure reduction needed
and the variations in the load. Pilot
Operated controls have been used
successfully with a pressure turndown
ratio as great as 20 to 1, and
could perhaps be used on a fairly
steady load from 100 psig to 5 psi.
The same valve would probably be
unstable on a variable load, reducing
from 40 to 2 psi.
There is no hard and fast
rule, but two valves in series will
usually provide more accurate
control. The second, or Low
Pressure valve, should give the
“fine control” with a modest turndown,
with due consideration
being given to valve sizes and
capacities. A practical approach
when selecting the turndown of
each valve, that results in smallest
most economical valves, is to
avoid having a non-critical drop in
the final valve, and stay close to
the recommended 10 to 1 turndown.
Series Installations
For correct operation of the
valves, some volume between
them is needed if stability is to be
achieved. A length of 50 pipe
diameters of the appropriately
sized pipe for the intermediate
pressure, or the equivalent volume
of larger diameter pipe is
often recommended.
It is important that the downstream
pressure sensing pipes
are connected to a straight section
of pipe 10 diameters
downstream from the nearest
elbow, tee, valve or other obstruction.
This sensing line should be
pitched to drain away from the
pressure pilot. If it is not possible
to arrange for this and to still connect
into the top of the
downstream pipe, the sensing
line can often be connected to the
side of the pipe instead.
Equally, the pipe between the
two reducing valves should
always be drained through a
stream trap, just as any riser
downstream of the pressure
reducing station should be
drained. The same applies where
a pressure reducing valve supplies
a control valve, and it is
essential that the connecting pipe
is drained upstream of the control
valve.
Bypasses
The use of bypass lines and
valves should usually be avoided.
Where they are fitted, the capacity
through the bypass should be
added to that through the wide
open reducing valve when sizing
relief valves. Bypass valves are
often found to be leaking steam
because of wiredrawing of the
seating faces when valves have
not been closed tightly.
If a genuine need exists for a
bypass because it is essential to
maintain the supply of steam,
even when a reducing valve has
developed some fault or is undergoing
maintenance, consideration
should be given to fitting a
reducing valve in the bypass line.
Sometimes the use of a parallel
reducing station of itself avoids
the need for bypasses.
Back Pressure Controls
A Back Pressure regulator or surplussing
valve is a derivative of a
pressure reducing valve, incorporating
a reverse acting pilot valve.
The pressure sensing pipe is connected
to the inlet piping so that the
pilot valve responds to upstream
pressure. Any increase in upstream
pressure then opens the reverse
acting pilot valve, causing the main
valve to open, while a fall below the
set pressure causes the main valve
to close down, Fig. II-18 (page 92).
These controls are useful in
flash steam recovery applications
when the supply of flash steam
may at times exceed the demand
for it. The BP control can then surplus
to atmosphere any excess
steam tending to increase the
pressure within the flash steam
recovery system, and maintains
the recovery pressure at the
required level.
The control is also useful in
eliminating non-essential loads in
any system that suffers undercapacity
at peak load times,
leaving essential loads on line.
Back Pressure Controls are
not Safety Valves and must never
be used to replace them.