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System Load Considerations
Large
buildings utilizing steam usually have complex multi-use,
multi-function application requirements. Applications
including seasonal heating, air conditioning, domestic hot
water, laundry and the like, as well as variations like time
of day and outdoor temperature all play a critical role in
load requirements. The difference between the maximum
capacity, when the building is fully occupied during the day
and at night when only minimally utilized, could be a factor
of 100 to 1 or more! Because the allowed for application loads
often are variable or completely non-existent at night. The
entire load on the steam supply at night might only be what is
created by condensation and trap losses in the piping
system. (Typical pilot operated PRV’s only have a rangeability
of 10:1).
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Safety
The final
stage of a typical pressure reducing station is responsible
for reliably supplying low pressure steam to the building
header for its various users. Safety relief valves can be
deployed but can be cost prohibitive in larger buildings due
to logistics and length of the vent pipe, not to mention code
constraints and testing requirements. Per the ASME B31.1 Code,
either safety relief valves are required on the final low
pressure stage OR if two stages of reduction are used
and both stages can be automatically set to the safe working
pressure of the equipment being served if the one of the
stages fail. For several reasons Warren Controls recommends
this latter solution and offers the Non-Indicating Safety
Pilot to first allow the primary stage to begin regulating to
low pressure should the secondary stage ever fail to do its
job. Conversely, the secondary stage has the actuator sized such
that it can handle the full drop in the event of primary stage
failure. Redundancy is assured. Best practices of protection
in managing failure of this final stage include the addition
of an automated safety shut-off valve with manual reset. A
separate pressure switch or safety alarm should be configured
to trip the automated safety shut-off valve. Many building
codes require an automated safety shut-off valve with manual
reset in systems that use manual valves to bypass a PRV (for
maintenance or manual recovery).
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Noise
With steam
there can be several aerodynamic noise producing mechanisms
including turbulent mixing noise, shock waves, impingement,
and shock turbulence interaction. Noise that exceeds
specifications can either be controlled at the source or along
the transmission path. Large pressure drops across a single
controlling device can aggravate noise situations. Another
benefit of dual stage reduction is that it is inherently less
noisy, splitting the pressure drop across two devices. The
noise level at the control valves will automatically be
calculated when using ValveWorks® to size the valves and
select the valves. Additional noise abatement and suppression
techniques and equipment can be deployed when control valves
exceed noise requirements so that overall system noise
requirements can be met.
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Reliability
A
typical building with multiple users means that with an outage
comes outrage. All energies are immediately put back into
getting the system back online. With this premise in mind, the
most practical design approach is to reasonably
assure that a
system failure does not occur in the first place and when one
does, reasonable recovery from failure is possible. Designing
in redundancy and recovery elements within the system as
described above is one aspect. Another is a choosing known
reliable components. When it comes to controls, pneumatic
controls when properly designed still represent the most
reliable types of controls in these high temperature
environments. Typical electronic controls reach their limit
before high incidence of failure at 122°F. Many steam
equipment rooms in the summer time are already at this
temperature near where the controls equipment would typically
be located.
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Features of the system
1. Control Valves
that
have been sized and selected for optimum performance by using
the Warren ValveWorks sizing program with Manual Override
handwheel on the Control valves that eliminate the need for a
by-pass line.
2.
Parallel
Flow Loops
on the
system in conjunction with inherently higher rangeability
globe style control valves with equal percent flow
characteristic trim can ensure system rangeability of greater
than 100:1.
3.
Two-Stage
Reduction
allows for
a method of redundancy as well as inherently minimizing
overall system noise by splitting the pressure drop across two
devices.
4.
Automated
shut-off valve
with manual reset used in place of a relief valve and vent
piping saves tremendous cost.
5.
Noise attenuation thru the use of static restrictors (mufflers) after
the control valves (when necessary) are far more effective
than special aerodynamic quiet trim designs and over the life
of the system, more reliable as quiet trim design are prone to
both clog with scale and erode faster.
6.
Safety
Limiting Pilots
(Warren 8351) that take control of the system should anything
happen to cause the system pressure to exceed the setting of
the low pressure indicating controller (Warren 8624).
7.
Indicating Pneumatic Controllers
(Warren
8624) with Gain and Integral Reset for accurate and dependable
control. These units are capable of operating in environments
that that would prohibit the use of electronic controls.
8.
Load Transfer Control.
Simple electric control that transfers the load from the
Low-Flow loop to the High-Flow loop as system demand changes.
9.
Alarm
Panel
10. Steam Traps
11. Isolation Valves
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Conclusions
Successful Steam Reduction Station design requires several
aspects for consideration. Start with the system codes you
must follow and a complete load analysis of the facility.
Quality of the steam and variations is supply pressure must
also be considered. Proper piping design and sizing is then
paramount. Once all factors are considered, Warren Controls
believes we can provide you with the highest performing and
most reliable system available. Contact your local Warren
Controls representative for further professional assistance.
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