Measuring pressure is not always as simple as it seems. In many cases, the sensing elements of pressure transmitters cannot come into direct contact with the process fluid for fear of corrosion, clogging of capillaries and product purity concerns. However, if the pressure measuring elements are not properly designed for the application, they can generate additional problems.
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Such measurements can be made, however, with remote seal transmitters. A remote seal transmitter includes a pressure transmitter, a remote seal and a capillary tube connecting the two. Together, they
transmit the pressure in the process fluid to the transmitter sensor indirectly.
Remote seal transmitters are used when it is necessary to isolate the transmitter from the process for any reason. The process fluid may be at an elevated temperature. The process material may be a slurry that would clog standard instrument lines. It may be corrosive. The process fluid may freeze or solidify during swings in ambient or process temperature. A food, beverage or pharmaceutical process may require a sanitary seal to avoid cavities and dead volume. Using remote seal transmitters facilitates easier cleaning to avoid contamination between batches.
One would also use this type of seal for applications in which the wet leg is not stable or often needs to be refilled, for density or interface measurements and for aggressive, corrosive, high viscosity, heterogeneous or toxic processes. Another application is for process fluids that may obstruct the connection or collect on the sensor, like crystallization steps and polymerization reactions.
Seals offer significant installation flexibility and maintenance advantages over wet leg systems. Remote seals make it easier to maintain the fluid between the tap and the transmitter, especially on the reference (low pressure) side. In vacuum systems, a closed seal system, rather than an open wet leg, maintains a constant height for the low side reference. The seals do not need to be refilled or drained. The seals are not susceptible to plugging or freezing; plus they can be easier to control than wet leg systems.
Parameters that determine the design of a remote seal transmitter are:
- Location of the transmitter (indoor versus outdoor).
- Temperatures (process and ambient).
- Exposure to vacuum.
- Pressure (operating and maximum).
- Length of capillary.
- Type of seal desired.
- Process connection, material of construction and similar considerations.
- Installation requirements.
- Measurement span.
A capillary filled with fluid transfers pressure from the seal to the transmitter. The length and the inside diameter of the capillary have an effect on the temperature effect and time response of the assembly. The most commonly used capillaries have diameters of 0.027 inch and 0.085 inch.
The most commonly used fill fluids are silicone oils*#151;DC-200 or DC-704. However, DC-704 is not recommended for use with nitric or sulfuric acids. CTFE (Inert Fill) is ideal for chlorine or oxygen service. Syltherm 800 primarily is used for process temperatures from 450 to 600 degrees Fahrenheit. Neobee M-20 is suitable for sanitary applications because it meets FDA Regulation 21CFR172.856.
The function of a gasket is to create and maintain a static seal between the two stationary, imperfect surfaces of a mechanical joint. The gasket must maintain this positive seal under the system operating conditions, including extreme temperature and pressure. The commonly used gasket is identified as C4401; the others are Teflon, graphite, Viton and Buna N.
The gasket must perform a number of jobs well. First, it must create an initial seal. Second, it must maintain that seal over time. Third, it must be easily removed and replaced. Success depends on the extent to which the gasket:
- Is impervious to the process fluid and fill fluid.
- Is sufficiently deformable to provide intimate contact between the gasket and the seating surfaces, yet resilient and resistant enough to creep to maintain an adequate portion of the applied bolt loading.
- Has sufficient strength to resist crushing under the applied load, resist blowout under system pressure and maintain its integrity when being handled and installed.
- Avoids contaminating the process fluid.
Type of remote seals available
Flush flange seals include a mounting flange that assists in aligning the diaphragm with the process connection. A flush flange seal also allows the use of a flushing ring between the diaphragm and the process flange.
Flush flange seals with lowers are used when an adapter is required between the seal and diaphragm so that larger diaphragms can be used on smaller process connections.
Extended flange seals provide a choice of extension lengths that mount on a pipe flange and extend in toward the tank wall. The diaphragm can, therefore, sit flush with the inner face of the vessel.
Pancake seals consist of a flat cylindrical body with a centered flush-mounted diaphragm. The seal is mounted to the process flange with the use of a backup (blind) flange. The capillary connection is on the side of the pancake seal, enabling its use if the space on the outside of the vessel is limited or restricted. Because the flange is not a permanent part of this seal, the flanges do not need to have identical pressure ratings. Also, the blind flange may be fabricated from a different material of construction since it is not exposed to the process fluid.
Threaded seals have a threaded process connection that allows easy mounting on threaded process pipe. The advantage of this type of seal is that the process pressure to be measured is applied over a large area, thus minimizing undesirable insensitive volumes or areas.
Chemical tee seals attach to a wedge flow element device and require the eight-bolt connection pattern. This one-size seal fits into any flow elements.
In accordance with 3-A standards, sanitary seals have smooth surfaces free of crevices in which bacteria or food may collect. This type of seal is used in food and beverage, biotechnology and pharmaceutical applications.
Because they are easy to remove, sanitary seals are appropriate for clean-in-place applications that require ease of disassembly and connection. However, this compatibility with cleaning materials needs to be taken into account. Sanitary seals attach to the process with clamps instead of bolts, as they’re designed for applications requiring ease of disassembly and connection.
An inline seal is suited for a rapidly flowing high-viscosity process fluid. The seal becomes an integral part of the process piping system, thus avoiding disturbing turbulence, corners, dead volumes or other flow imperfections. The seal is a cylindrical thin-walled diaphragm encased in a cylindrical body. This construction allows for extremely accurate pressure measurements. The undisturbed process flow produces a self-cleaning effect in the measurement system.
Replaceable, non-welded diaphragms are undesirable because even the slightest loss of fill fluid drastically affects transmitter performance. It is virtually impossible to keep a replaceable diaphragm leak-tight. Teflon diaphragms should never be used. In addition to leaking, they are non-elastic and subject to cold flow. These characteristics make achieving a repeatable measurement almost impossible.
Ways to reduce measurement errors
Keep capillary lengths as short as possible. Never mount seals and capillaries in direct sunlight. Use capillaries of the same length on both taps when measuring differential pressure. Be sure that both capillaries experience the same temperature. For instance, avoid installing one capillary in a shady area and the other in the sun. This eliminates zero shift that results from unequal thermal expansion and contraction of the fill fluid in the high and low legs.
The most common application of diaphragm seals is for level measurement. Proper ranging of a transmitter for level service requires considering the specific gravity of both the fill fluid and the process fluid. With dual-leg pressure transmitters, mismatched fill volumes (unequal lengths of capillary), degree of mismatch between seals (same sizes tend to offset any unbalance, though not completely) or uneven temperatures on seals and capillaries can affect the readings. Seals with a large measuring surface provide better results. The amount of error attributable to significant changes in either the process or ambient temperature varies widely with the combination of seal, capillary and fill fluid.