2000 Hook-up Book - Spirax Sarco

SYSTEM DESIGN
30
Temperature Control Valves for Liquid Service
Mixing And Diverting
Three-Port Valves
A three-port temperature control
has one port that is constantly
open and it is important that the
circulating pump is always positioned
on this side of the system.
This will prevent the risk of pumping
against dead end conditions
and allow correct circulation to do
its job.
The valve can be used either
to mix or divert depending upon
how it is piped into the system. A
mixing valve has two inlets and
one outlet, a diverting valve has
one inlet and two outlets.
Fig. 43 illustrates the threeport
valve used as a mixing valve
in a closed circuit. It has two inlets
(X and Z) and one outlet (O) which
is the permanently open port. Port
X is the port open on startup from
cold, while Port Z will normally be
closed on startup from cold. The
amounts of opening in Ports X and
Z will be varied to maintain a constant
outlet temperature from Port
O. Thus a certain percentage of
hot boiler flow water will enter
through Port X to mix with a corresponding
percentage of cooler
return water via Port Z.
When the three-port valve is
used to blend cold supply water
with hot water which may be from
Hydrogen gas is an important ingredient to many oil refining
processes. Large multi-stage compressors are located
in operating sections throughout the refinery. Considerable
attention is paid to maintaining gas quality, and keeping
liquid from accumulating in the system.
The telltale signs of entrained liquid became evident
as a high-pitched whistling noise was heard coming from
the compressor sections. It was determined to be the
result of poor cooling water temperature control. The cooling
water/Glycol mixture leaving the heat exchanger at
95°F, circulating through the compressor jacket was causing
excess hydrogen condensing on the cold surfaces of
jacket walls. It’s important to maintain the 95°F heat
exchanger outlet temperature to assure that sufficientlycool
water/Glycol is supplied to the compressor sections
necessary for proper heat transfer.
another source, for use in showers
or similar open circuits where
all the water does not recirculate,
it is essential that the pressure of
the supplies be equal. For these
applications, it is recommended
that both the X and Z ports be fitted
with check valves to prevent
any scalding or other harmful
back-flow condition.
With the valve connected as
shown in Fig. 43A, we now have a
diverting arrangement. The valve
has one inlet and two outlets. Hot
water enters Port O and is either
allowed through Port X to the
equipment or through Port Z to
return to the boiler.
The Need for Balancing
The action of a three-port valve in
a closed circuit system, whether
mixing or diverting, tends to
change the pressure conditions
around the system much less than
does a two-port valve. This stability
is increased greatly when a
balancing valve is fitted in the
bypass (or mixing connection)
line. Not fitting a flow balancing
valve may result in short circuiting
and starvation of other subcircuits.
The balancing valve is set so
that the resistance to flow in the
bypass line equals or exceeds that
in the load part of the subcircuit.
In Fig. 43, the balance valve
must be set so that the resistance
to flow in line B-Z is equal to the
resistance to flow in line B-A-X. In
Fig. 43A, resistance B-Z must
equal resistance X-C-B.
Makeup Air Heating Coils
Air heating coils in vented condensate
return systems,
especially preheat coils supplied
with low pressure steam modulated
by a control valve, can present
difficulties in achieving satisfactory
drainage of condensate. There
is no problem at full load with
properly designed equipment, but
part load conditions often lead to
flooding of the coils with condensate,
followed by waterhammer,
corrosion and sometimes by
freeze-up damage. These problems
are so widespread that it is
worth examining their causes and
remedies in some detail.
Coil Configurations
The coils themselves are usually
built with a steam header and a
condensate header joined by
finned tubes. The headers may be
both at one side of the unit, with
hairpin or U tubes between them,
or sometimes an internal steam
tube is used to carry the steam to
the remote end of an outer finned
tube. Vertical headers may be
used with horizontal finned tubes,
Case in Action: Hydrogen Compressor Cooling Jacket Temperature Control
Solution
A 2" temperature control with adjustable bleed and a sensing
system was installed on the cooling water/Glycol outlet
piping from three stages for each of two compressors.
They were set to maintain a discharge temperature of
140°F. This had the effect of holding back Glycol in the
jacket sufficiently to prevent excess hydrogen condensing
while, at the same time, maintaining necessary cooling.
Benefits
• Reduced energy consumption as hydrogen condensing
is reduced.
• Installation of a self-contained control was far less
expensive than a more sophisticated pneumatic type
that was also under consideration.
• System start-up was fast because of the easily-adjusted,
pre-calibrated sensing system.
• Accurate process temperature control of each jacket
resulted from having separate controls on each.