High-Pressure
Drain Service
FCD ADAMS8014-00
The Problem
The small valves presently used in high-pressure steam drain
service exhibit severe seating wear. This condition leads to continuous and increasing steam leakage which requires frequent valve
replacement. The end result is operating and maintenance costs that
are higher than necessary.
Typical Service
In a typical application, the before-seat cavity drains from turbine
stop and control valves which are normally 1" to 2" in size and
designed to prevent water induction on startup. These drains are
usually remote-actuated (motor or pneumatic). They are opened
upon shutdown or trip and, in some cases, are subjected to system
blowdown from over 3500 psi (supercritical plants) for several
minutes. Upon normal startup, drains remain open until condensation is blown clear and some superheat achieved. Valve closure is
made normally at 700-1000 psig line pressure at the time the unit
is synchronized. Operation during startup results in highly erosive
wet steam running through the valve for several hours at relatively
high-pressure drops.
Drawbacks of Standard
System Configuration
The high-pressure drop combined with low-quality steam rapidly
erodes the drain valve seat, causing loss of tight shutoff. In
typical single-valve installations (Figure A), the entire pressure
drop is taken across the valve seat, usually setting up damaging
choked flow conditions in the seat area. Also, because there is no
backup isolation capability, leaking drains result in costly energy
loss during operation.
Single-valve drain systems are designed by turbine manufacturers
and engineers as a cost compromise. Often, space is limited, making in-line repair difficult, if not impossible. Complete replacement is
often the only reasonable approach—and the cycle starts all over.
Two other factors enter into the failure analysis. First, Y-globe valves
are most often used as drains and are subject to the problems cited
in Anchor/Darling Valve’s “Isolation Valve Application: Eliminating
High-Pressure Valve Leaks.” Secondly, to reduce high drain flows
in the latter stages of startup, operators often throttle drain valves,
thereby accelerating seat wear.
Solutions
There are several possible solutions to the drain problem. The
“right” solution is dependent on the existing system configuration
and a combination of the correct components with sound piping
design to achieve the most cost-effective result.
A. Two-Valve System (Figure B) – An Anchor/Darling Series
700 Double-Disc Gate Valve may be installed upstream of the
existing drain valve, strictly to provide leak-tight shutoff. The
drain valve may be used for throttling flow (acting as a variable
orifice). This option has the advantages of utilizing existing piping and drain valve, placing the tight shutoff valve upstream of
the throttling valve, thus avoiding high velocities and turbulence,
and minimal cost impact. The disadvantage may be insufficient
space for installation or maintenance of the new gate valve. For
this system, when opening the drain line, the gate valve should
be opened first, followed by opening the globe valve. When
closing the drain line, the globe valve should be closed first,
followed by closing the gate valve.
���������������������
B. Single Valve with Orifice (Figure C) – A single Series 700 Gate
Valve and a fixed orifice, either single or multi-stage (for noise
control), may replace the standard drain valve. The advantages
of this arrangement are simplicity of maintenance and economy
of space while taking the pressure drop across the orifice and
minimizing valve wear. The orifice assembly should be placed
downstream from the valve a minimum of ten pipe diameters.
The disadvantages are that the orifice must be larger than ¾"
to comply with ASME guidelines*, or it may be susceptible to
plugging. Throttling of flow is not permitted (nor should any be
necessary). Finally, a larger discharge line should be considered
downstream of the orifice to obtain lower velocities.
C. Two Valves with a Fixed Orifice (Figure D) – Combines options A
and B, meeting the requirements for minimum size orifice, while
Figure A
Figure B
Main Steam
Main Steam
Motor or Air-Actuated Globe Drain
Valve (Normal flow
under the seat, which
is appropriate)
providing the longevity and simplicity of an optimally sized fixed
orifice. (All parts can be incorporated into one prefabricated
assembly.) The disadvantage is the higher relative cost.
Summary
Whereas the cost of any of the proposed options exceeds the original equipment single-valve arrangement, the difference will be offset
in the dramatic savings affected by lower maintenance and lost
energy costs. Actual installations using Anchor/Darling Series 700
Double-Disc Gate Valves have provided excellent results.
*ASME Standard TDP-1-1980 (Section 3.3-3.6)
Figure C
Figure D
Main Steam
Anchor/Darling
Double-Disc Gate
Valve (Motor or airactuated optional)
Drain Valve
To Condenser
To Condenser
Main Steam
Anchor/Darling
Double-Disc
Gate Valve
Anchor/Darling
Double-Disc
Gate Valve
Fixed Orifice –
Single or
Multi-Stage
Fixed Orifice
To Condenser
Globe
Bypass
Valve
To Condenser
Flowserve Corporation has established industry leadership in the design and manufacture of its products. When properly selected, this Flowserve product is designed to perform
its intended function safely during its useful life. However, the purchaser or user of Flowserve products should be aware that Flowserve products might be used in numerous
applications under a wide variety of industrial service conditions. Although Flowserve can (and often does) provide general guidelines, it cannot provide specific data and warnings
for all possible applications. The purchaser/user must therefore assume the ultimate responsibility for the proper sizing and selection, installation, operation, and maintenance of
Flowserve products. The purchaser/user should read and understand the Installation Operation Maintenance (IOM) instructions included with the product, and train its employees
and contractors in the safe use of Flowserve products in connection with the specific application.
While the information and specifications contained in this literature are believed to be accurate, they are supplied for informative purposes only and should not be considered
certified or as a guarantee of satisfactory results by reliance thereon. Nothing contained herein is to be construed as a warranty or guarantee, express or implied, regarding any
matter with respect to this product. Because Flowserve is continually improving and upgrading its product design, the specifications, dimensions and information contained herein
are subject to change without notice. Should any question arise concerning these provisions, the purchaser/user should contact Flowserve Corporation at any one of its worldwide
operations or offices.
For more information about Flowserve Corporation, visit www.flowserve.com or call USA 1-800-225-6989.
���������������������
© 2004 Flowserve Corporation, Irving, Texas, USA. Flowserve is a registered trademark of Flowserve Corporation.
(Part HPDS)
FCD ADAMS8014-00 Printed in USA.