SPECIAL PRINT

Seat Leakage Detection
Special print from SAMSON MAGAZINE
11/2013

SAMSON AG
Monika Schneider
Technical Documentation

SMART IN FLOW CONTROL.

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Seat Leakage Detection
Special print from SAMSON MAGAZINE, 11/2013

Precise Seat Leakage Detection in Control Valves
If a valve‘s tight shut-off is impaired, the seat leakage does not become visible on the outside. Yet, it can be a problem on the inside.
If the seat and plug do not shut off properly, an unacceptable amount of the process medium continues to flow through the valve
even though it is supposed to be fully closed. The medium always finds a way between the seat and plug. An immediate consequence is that the control accuracy deteriorates. In the long run, the valve‘s service life is shortened. By connecting a sensor to theSeries 3730 Positioners with integrated diagnostics, SAMSON has found a way to reliably detect and indicate leakage inside thevalve.
Seat leakage
In a valve, the plug position determines the cross-sectional areaof flow between the seat and plug, and thus the flow rate
through the valve. When the valve is closed, the plug rests on
the seat and, ideally, leaves no space for the process medium
to flow through. The actual leakage rate of a valve depends onthe valve design, particularly on the shape of the plug and thesealing at the seat. Further determining factors are the positioning force of the actuator and the pressure drop that exists between the valve inlet and outlet. Through the course of a valve‘sservice life, the medium flow through the seat and plug whenthe valve is closed increases due to progressing erosion at theplug caused by abrasive media, cavitation or flashing. As aresult, the valve no longer complies with the specifications forwhich it was sized originally. In practice, a small seat leakage
rate is tolerated in most control applications. In shut-off valves,
however, even the smallest amount of unwanted medium flow
can cause severe damage, for example when toxic, highly
reactive or highly flammable media get into the process without
being properly controlled. In such cases, it is vital that signs ofwear on the plug are detected at an early stage.
Flow-induced soundEven the smallest medium flow creates sound of a measurable
level. SAMSON uses this phenomenon to detect valve leakage at
an early stage. A leakage sensor mounted on a pneumatic control valve can reveal and indicate impaired valve shut-off with thehelp of the EXPERTplus valve diagnostics integrated into the posi-2tioner. As a result, the combination of leakage sensor and positioner with diagnostic functions allows operators to precisely
monitor their valves for seat leakage. Monitoring the leakage
rate is indispensable particularly in safety-instrumented systems
where valves must shut off on demand, reliably and without leakage. In such applications, plant operators must be alerted whenunacceptable amounts of the medium flow through a closed valve.
So far, mainly flow meters have been used for seat leakage monitoring. To detect minimal leakage rates, however, it makes sense
to supplement the flow meters with leakage sensors, which require neither an additional power supply nor a connection to theprocess control system. Mr. Marcel Richter, product manager at
SAMSON for positioners, the integrated EXPERTplus valve diagnostics and leakage sensors, sees the main benefit for customers
in the fact that the positioner-leakage sensor combination can be
integrated seamlessly into existing systems: "The leakage sensor
is powered and linked to the process control system simply by
being connected to the positioner. Whether the measured soundpressure level is transmitted to the control system using the HART®
or FOUNDATION™ fieldbus protocol depends on the selected
positioner. This also means that only one leakage sensor version
is required for the different communication protocols employedby the positioners."
Prime valve performance
A valve setup matching the actual conditions in a plant has
been installed on the test bench at SAMSON‘s Frankfurt

headquarters to show how well the unit comprising a control
valve with positioner, integrated diagnostics and leakage
sensor works in the field. One of the visitors impressed by thesetup was Mr. Rolf Ohrndorf, head of SAMSON‘s Cologne
engineering and sales office: "For those of us interacting
directly with the customers, this is a good opportunity to see
the leakage sensor firsthand and learn how it works. I wasparticularly impressed by the low threshold that caused theleakage rate to be indicated in the valve diagnostics."
Mr. Ohrndorf considers the test bench setup a valuable presentation for SAMSON staff and customers alike, even though
the focus is on the sensor‘s functioning. Other benefits become
evident nonetheless, for example that the solution with a leakage sensor is cost-effective and easy to install.
Mounting the leakage sensor to the valve is simple. It isscrewed onto the valve and connected to the positioner. Thatis all it takes before the first measurements can be performed.
The leakage sensor measures the sound pressure level createdby the medium flowing through the valve every time the valve
is fully closed. The assessment necessary for leakage detection
is done by the positioner, or more precisely the EXPERTplus
valve diagnostics integrated into the positioner. They save andanalyze the transmitted sound pressure level. Based on areference measurement performed when the valve wasclosed, the positioner generates an alarm every time the valve
is closed tightly and the customizable alarm limits for theleakage rate are exceeded. The urgency of the alarm can also
be defined by the operator. The color-coded classification isFor example soundpressure level, condensed
state NE 107,valve position, ...

Supported by diagnostics
On the test bench, the leakage sensor is currently used withthe latest positioner from the Series 3730. The Type 3730-6
communicates with the process control system using theHART® protocol. Two of the four HART® variables can beused to permanently transmit the sound pressure level andthe status of EXPERTplus. In this way, the valve‘s shut-off isproperly monitored at all times. The leakage sensor further
enhances safety-instrumented shut-off valves that are operated very rarely but expected to either close or open on
demand (fail-close or fail-open applications). In fail-open
applications, the valve is closed during normal operation,
which means that the leakage sensor monitors the leakage
rate constantly. In fail-close applications, the valve is open
during normal operation. In such applications, the leakage
sensor is mainly needed to perform the mandatory functional tests at regular intervals.
As part of these mandatory proof tests, the valve is moved
through its entire travel range from its normal operating position (open) to its fail-safe position (closed) while key data,such as the breakaway and closing times, are recorded. A
leakage sensor additionally provides information on whether the valve is still able to shut off reliably and tightly after
remaining in the open position for longer periods of time.

Process control
stationFlow rate

F

Leakage sensor

based on NAMUR Recommendation NE 107 and indicatesthe valve‘s diagnostic status as either green (no message),yellow (out of specification) or red (failure) depending on thesettings adjusted by the operator. After his visit to the Frankfurt
test bench, it was clear to Mr. Ohrndorf that the leakage sensor
presented a cost-effective solution for plant operators todetect impaired valve shut-off: "I think our customers willrecognize the potential offered by the sensor, particularly forsafety-instrumented systems where downstream flow meters
may reach their measuring limits."

Mr. Richter is convinced that, with the positioner and leakage
sensor, SAMSON has introduced a high-quality product tothe market that complies with the latest technical requirements. "Visitors of the test bench can see for themselves. For
our international staff as well as our customers who do nothave the chance to see the leakage sensor at work firsthand, we have uploaded a product video to the Internet. It
shows how precisely the leakage sensor works."

Type 3241-7 with Type 3730-6

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SPECIAL PRINT

Seat Leakage Detection

SAMSON AKTIENGESELLSCHAFT
Weismuellerstrasse 3 · 60314 Frankfurt am Main, Germany
Phone: +49 69 4009-0 · Fax: +49 69 4009-1507
E-mail: samson@samson.de · Internet: www.samson.de4
SMART IN FLOW CONTROL.

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