By: Instrument Guru, December 2, 2014
Pressure gauges are often used in extreme conditions in industrial processes, and some gauges are specifically designed for use in these difficult process environments. These tough process gauges typically include features such as hardened housings, liquid-filled casings, and a stress-reducing Bourdon tube design, and they are often equipped with a diaphragm seal to prevent contact between the hot or corrosive media and the gauge itself. A diaphragm seal protects pressure gauges from dangerously hot, viscous, or corrosive media and dramatically extends the lifetime of the instrument.
Understanding diaphragm seals
A diaphragm seal is connected to a gauge as either a direct connection or by way of a capillary. An elastomer or flexible metal diaphragm keeps the process media from contacting the workings of the gauge. The chamber between the diaphragm and the gauge is filled with a fluid that accurately transfers the pressure of the process media by hydraulic means. Any change in process media pressure is transmitted through the flexible diaphragm into the system fill fluid, which then connects to the Bourdon tube of the gauge.
Diaphragm seals can be easily mounted onto gauges using threaded, flanged, in line, sanitary, or other types of connections.
In most cases, diaphragm seal bodies and the diaphragms themselves are stainless steel, but instrument industry leaders like WIKA also produce diaphragm seals from other materials including carbon steel and Hastelloy® C-276 for specific applications. WIKA’s diaphragm seals are designed to operate in pressure applications from 10″ Hg to 20,000 psi and at media temperatures between -130°F and +752°F.
Diaphragm seals are also installed on pressure transmitters and switches exposed to extreme conditions.
When to use a diaphragm seal
- Protective diaphragm seals are typically used in applications where direct contact with the process media will degrade the instrument. Diaphragm seals should be considered under conditions including:
- High-temperature media – A diaphragm can be designed to include a cooling element; for example, a diaphragm seal with an 8″ cooling tower can tolerate temperatures up to 750°F.
- Corrosive media – WIKA can provide diaphragm seals made of a variety of custom alloys, including PTFE, gold and silver, or carbon steel. WIKA’s Metal Bonded Diaphragms are designed for processes requiring Hastelloy®, Monel® and Tantalum flush diaphragms.
- Viscous or crystallizing media – Using pressure seals reduces the number of connections in a process, minimizing internal cavities where clogs typically form.
- Media with suspended solids – Diaphragm seals minimize clogging in process applications such as pharmaceutical, pulp and paper, food and beverage, chemical, and waste water.
- When overpressure protection is required – WIKA has designed a diaphragm seal with a continuous-duty contoured diaphragm bed for robust overpressure protection.
- When sanitary cleanliness is required – WIKA’s patented InLine Seal™ makes sure that your process system does not develop “dead spaces” that encourage bacterial growth.
- Highly toxic or dangerous media – An all-welded process gauge and diaphragm seal system offers extra protection to minimize the chance of any toxic media escaping.
Top quality WIKA diaphragm seals specifically designed around your processes can dramatically extend the lifetime of your pressure measurement instrumentation and lead to significant cost savings. Contact us to learn more about how a small investment in the right diaphragm seals pays off over the long run.
Forberg Scientific, Inc.
P: 855-288-5330
E: mechanicalsales@forberg.com
www.forberg.com
Inadequate attention can make plants vulnerable to mishaps
From: WIKA Jason Deane
The simple pressure gauge
is an often-overlooked defense mechanism for preventing accidents. However, in
auditing more than 250 plants, WIKA Instrument discovered that up to 25% of all
pressure gauges were broken, damaged or misapplied — this represents an average
of eight deficient gauges located within 20 feet of each employee.
A failed gauge compromises
a plant's ability to detect a problem before a safety incident occurs.
Malfunctioning gauges also can lead to media leaks, fugitive emissions and a
fire or explosion, taking a toll on safety and reliability.
Even minor accidents can
cause employee injury and lead to downtime. Any accident or leakage also puts
staff sent to fix the problem into harm's way, which, of course, can lead to
further employee injury and lost hours.
Many causes contribute to this dangerous situation with gauges. Fortunately,
they can be prevented.
CAUSES OF GAUGE FAILURE
Through its evaluation of
more than 150,000 gauge installations, WIKA has identified eight common causes
of failure. So, let's look at each, along with the solution.
1. Vibration. Many
pieces of equipment vibrate. However, excessive vibration can lead to gauge
failure and may indicate a problem with a component. Solution: install a gauge
that will resist vibration better — i.e., a liquid-filled or direct-drive gauge
with only a single moving part.
2. Pulsation.
A rapidly cycling medium within a pressure system can make a gauge pointer move
erratically and eventually can lead to breakdown of internal parts. Solution:
install a restrictor and liquid-filled case to dampen pulses on a standard gauge or replace with a direct-drive gauge that lacks gears and linkages.
3. Temperature.
Extreme temperatures cause sweating and loosening in metal joints and
eventually can cause them to crack. Solution: install a gauge with a fully
welded diaphragm seal and consider adding an on-board cooling element to combat
the highest temperatures.
4. Overpressure and
pressure spikes. Frequent pegging against the stop pin can bend
the gauge pointer and compromise the integrity of the Bourdon tube or sensing
element and, ultimately, lead to rupture. Solution: install an overpressure
protector to inhibit readings that exceed gauge capacity.
5. Corrosion.
The highly corrosive media often found in process plants can damage the sensing
material in gauges. Solution: install a diaphragm seal that's constructed from
material that will withstand the corrosive.
6. Clogging.
A medium that contains suspended particles or is viscous or can crystallize can
clog the pressure system and make gauge readings unreliable. Solution: install
a diaphragm seal with a clog-preventing barrier.
7. Steam.
Some media produce steam or other vapors that can damage the internal parts of
gauges. Solution: install either a mini-siphon with an internal chamber to
reduce surges or a full siphon, making sure to include a coil for horizontal
applications and a pigtail for vertical ones.
8. Mishandling and abuse.
Even properly installed gauges will start to malfunction if mistreated over
time. Solution: conduct regular safety and maintenance training for all
employees who come into contact with or proximity to gauges.
Unfortunately, many plant
personnel aren't properly equipped or experienced enough to recognize and
address all these problems. That, however, doesn't reduce the importance of
doing so.
TACKLING THE PROBLEMS
When beginning to address
instrument shortcomings, keep in mind that studies show that fewer than 0.25%
of piping components account for greater than 80% of controllable fugitive
emissions. Installing gauges with welded diaphragm seals on these components
creates a dual containment device, which is required by the U.S. Environmental
Protection Agency. This means plants can correct a major source of violations
and fines by addressing a very small percentage of connection points. For many
facilities, this is an excellent place to start to get meaningful results
quickly.
Another fairly straightforward step that's simple to implement but can yield
powerful results is standardization of gauges. This reduces the number of
replacement parts that must be kept in inventory — and confusion by
technicians. In other words, when replacing an old or faulty gauge, employees
more likely will select the correct gauge rather than resorting to like and
kind replacement. This also helps ensure the storeroom maintains proper
inventory, helping cut costs.
Plants that don't have the resources to identify and correct faulty and
misapplied pressure measurement instruments can get outside help, such as from
WIKA's FAST Team. Any audit team should:
• Visually evaluate the
plant's gauge population and look for issues that need to be addressed.
• Diagnose gauges that pose threats and uncover the causes.
• Formulate a strategic plan to address all the discovered issues.
• Audit the storeroom and streamline inventory, reducing redundant part numbers
and guesswork.
• Provide dependable processes to prevent misapplying instruments in the
future, and coordinate employee-training programs.
Given the complexity of
managing the operations of a process plant, it's easy to understand how smaller
components such as mechanical pressure instruments can be overlooked. However,
using gauges as early warning devices can improve uptime, safety and profits.
Money spent on the humble gauge very well could be the best investment a
processing plant can make.
If you would like more information, pricing and availability on WIKA Pressure Gauges or other WIKA Instrument products please contact Forberg Scientific Customer Service.
Tool Free: 855-288-5330Email: mechanicalsales@forberg.com
When selecting a pressure gauge, it is important to consider the following factors to ensure safety and accuracy:
1. Pressure fluid composition
2. Pressure fluid temperature
3. Ambient conditions
4. Pressure range
5. Conditions affecting wear of the system
6. Method of mounting
7. Required accuracy
1. Pressure fluid composition
Since the sensing element of a pressure gauge may be exposed directly to the measured medium, consider the characteristics of this medium. It may be corrosive, it may solidify at various temperatures or it may contain solids that will leave deposits inside the sensing element. For pressure fluids that will not solidify under normal conditions or leave deposits, a Bourdon tube gauge is acceptable. Otherwise a Sealgauge or diaphragm seal should be used. A chemical compatibility chart follows this section to aid in the selection of the proper sensing element material.
2. Pressure fluid temperature
Steam and other hot media may raise the temperature of the gauge components above safe working limits of the sealed joints. In these cases it is recommended that a siphon, cooling tower or diaphragm seal be used in conjunction with the pressure gauge.
3. Ambient conditions
The normal ambient temperature range for WIKA pressure gauges is -40F to +140F (-4C to +60C) for dry or silicone-filled gauges and -4F to +140F (-20C to +60C) for glycerine-filled gauges. The error caused by temperature changes is +0.3% or -0.3% per 18F rise or fall, respectively. The reference temperature is 70F (20C). The correction is for the temperature of the gauge and not the temperature of the measured medium. Remote gauge mounting using a diaphragm seal and capillary line is one alternative for applications involving extreme ambient temperature. Moisture and weather effects must also be considered. Liquid-filled gauges prevent condensation build up. For outdoor use, stainless steel, brass or plastic cased gauges are recommended.
4. Pressure range
A gauge range of twice the working pressure is generally selected. The working pressure in all cases should be limited to 75% of the gauge range. Where alternating pressure and pulsation are encountered, working pressure should be limited to 2/3 of the gauge range.
5. Conditions affecting wear of the system
In applications involving severe pressure fluctuation or pulsation, the use of restrictors and/or snubbers is recommended. In addition, liquid-filled gauges increase the service life of gauges in these conditions. WIKA liquid filled gauges are generally filled with glycerine. Silicone for larger temperature extremes and Halocarbon® for use with oxidizing agents such as chlorine, oxygen and hydrogen peroxide are also available.
6. Method of mounting
Radial (LM) and back (CBM or LBM) connections are available for most WIKA gauges. WIKA stocks gauges with standard NPT threaded connections. Other types such as metric threads, straight threads, hose barbs and special fittings are available as a special order. Pressure gauges should be mounted in the upright position. For applications where the gauge is mounted side ways, horizontally or upside down, contact WIKA or Forberg Scientific Inc. Customer Service for gauge type compatibility.
7. Required accuracyWIKA stocks gauges with accuracies from ± 3/2/3% to ±0.1% of span (ASME Grade B to Grade 4A).
To ensure safe and accurate gauge selection, you must take all of the above factors into consideration. When in doubt, please do not hesitate to contact Forberg Scientific Inc. Customer Service for assistance!
Toll Free: 855-288-5330
Email:
mechanicalsales@forberg.com
Selection Guidelines
When selecting a diaphragm seal assembly, the following details must be taken into consideration to ensure a safe
and satisfactory operation. For specific technical assistance regarding temperature effects, volumetric compatibility, etc.,
contact the Forberg Scientific customer service department or send a completed diaphragm seal specification sheet to the factory
for analysis.
- Process Composition
- Temperature
- Pressure Range
- Pressure Instrument
- Process Connection
- System Fill Fluid
- Mounting Positions
- Response Time
- Seal and Gauge Matches
1. Process composition
Since the diaphragm and lower housing of the diaphragm seal will be exposed to the process medium, it is critical to
select materials for these components which will be compatible with this medium. Tables are available to assist in the
selection of these materials (see Pressure Gauge Section); however, the customer is the ultimate source for specifying suitable
materials. WIKA cannot guarantee suitability. For information, see numerous reference guides such as corrosion table
reference books. If the pressure fluid is very thick, solidifies, or is full of solids, this should also be taken into consideration.
2. Temperature
Each diaphragm seal measurement system (diaphragm seal, pressure instrument, and cooling element or capillary, if
applicable) is filled with an amount of fill fluid at an ambient temperature of about 70oF. This temperature is referred to as
the system fill temperature. The fill fluid will expand or contract according to temperature changes. This in turn causes
the pressure in the sensing element to rise or fall, thus adding zero shifting effects to the instrument output. To reduce
this effect, the temperatures of the process and the environment should be specified when selecting a diaphragm seal
system (see Diaphragm Seal Specification Sheet). Special advanced calibration techniques can be used to ensure the
best possible accuracy. At temperatures above 300 F, a cooling element or capillary is suggested to protect the pressure
instrument.
3. Pressure range
The displacement volume on the diaphragm seal required to "drive" each diaphragm seal measurement system
(diaphragm seal, pressure instrument and capillary, if applicable) must be greater than the displacement volume needed
to move the pressure sensing element. Normally, the lower the pressure range, the larger the diaphragm is required to
"drive" the system. Conversely, for higher pressure ranges, smaller diaphragms are sufficient. Pressure transmitters also
follow the general rule of the lower the pressure, the larger the diaphragm required.
4. Pressure instrument
As mentioned above (Item 3 - Pressure range), the diaphragm seal must supply sufficient displacement volume to
enable the pressure instrument to reach full scale. As a general rule, smaller size gauges are better suited to low pressure
applications since less displacement volume is required on the part of the diaphragm seal to drive the pressure instrument.
5. Process connection
The process connection is specified by the customer. Most process connections are threaded, flanged, or clamped;
however, additional connections are available. Teflon® coating and lining is only available in flanged connections, since
tapered NPT threads strip off the Teflon® during installation. However, solid Teflon® threaded connections are available
with NPT threads.
6. System fill fluid
WIKA offers a wide range of system filling fluids allowing temperatures from -130 F to 752 F. Chemical compatibility of the
system fill fluid with the process fluid must be carefully considered in the event of a leak. In food processing applications a
nontoxic fluid should be selected. Special fill fluids are also available for oxidizing media such as oxygen and chlorine.
7. Mounting position
Mounting position is important for diaphragm seal systems which include a capillary. The level difference between the
diaphragm seal and the pressure instrument causes a hydrostatic pressure to act on the sensing element:
a. For gauges mounted above the level of the diaphragm seal, the pointer on the dial
of the gauge will be lower than the zero point.
b. For gauges mounted below the level of the diaphragm seal, the pointer on the dial
of the gauge will be higher than the zero point.
The diaphragm seal system can be calibrated to compensate for the effect caused by the hydrostatic pressure,
if the level difference is known in advance.
8. Response time
Response time, i.e., the time it takes the pressure instrument to indicate 90% of the value of a sudden pressure
variation, is especially important for instrument/diaphragm seal assemblies which include a capillary. Response time
increases significantly in systems with long capillaries. In applications requiring long capillaries, response times can be
reduced by using larger diameter capillary tubing and reducing the viscosity of the system fill fluid. Be advised that
increasing the inner diameter of the capillary increases the temperature influence of the measuring system. Forberg Scientific can consult WIKA if detailed information is needed.
9. Seal and gauge matches
For low ranges, gauge preference is 2XX.54 or 2XX.34 for access to perform calibration adjustments. Gauges with crimp
rings might not be usable due to potential recalibration. The table below shows the common matches between gauge
and diaphragm seal types recommended by the WIKA.
If you would like additional information about
WIKA Diaphragm Seals
or other WIKA Products please
contact Forberg Scientific, Inc.
Toll Free:
855-288-5330
Email: mechanicalsales@forberg.com
Diaphragm seals, also referred to as chemical seals, are used to isolate pressure gauges, switches, and transmitters from clogging and/or corrosive media. Standard diaphragm seal bodies and diaphragms are made of stainless steel; however, a variety of materials from carbon steel to Hastelloy® C-276 are available to meet the demands of most applications. WIKA diaphragm seals can operate in pressure applications from 10" H2O to 20,000 psi and media temperature between -130°F and 752°F.
EXAMPLES OF TYPICAL DIAPHRAGM SEAL APPLICATIONS
-
The media is corrosive and may damage a sensitive element such as a Bourdon tube gauge, pressure switch or transmitter diaphragm.
- The temperature of the media may be too high for a standard gauge, switch or transmitter to operate properly.
- The media is highly viscous or tends to crystallize, or polymerize and may clog the pressure port of a gauge, switch or transmitter.
- The media is non-homogenous or contains suspended matter such as wood pulp which may clog the pressure port of a gauge, switch or transmitter.
- Remote reading is required. A diaphragm seal with a capillary line will allow remote installation of a pressure instrument.
- The sanitary cleanliness level is critical. A flush mounted or INLINE SEAL™ sanitary type diaphragm seal avoids dead space and cavities.
- The media is toxic or hazardous and may pollute the environment. A suitably designed diaphragm seal will provide additional protection.
- The application requires high overpressure protection. A diaphragm seal with a contoured diaphragm bed can be configured to provide overpressure protection and protection to the instrument.
WIKA diaphragm seal systems are an excellent value and offer savings by:
- Meeting fugitive emission requirements
- Extending the service life of the pressure instrument
- Reducing the cost of installation
- Reducing or eliminating maintenance costs
If you would like additional information about WIKA Diaphragm Seals or other WIKA Products please contact Forberg Scientific, Inc.
Toll Free: 855-288-5330
Email: mechanicalsales@forberg.com
Pressure gauges are often used in extreme conditions in industrial processes, and some gauges are specifically designed for use in these difficult process environments. These tough process gauges typically include features such as hardened housings, liquid-filled casings, and a stress-reducing Bourdon tube design, and they are often equipped with a diaphragm seal to prevent contact between the hot or corrosive media and the gauge itself. A diaphragm seal protects pressure gauges from dangerously hot, viscous, or corrosive media and dramatically extends the lifetime of the instrument.
