PRESSURE TRANSMITTER:

INTRODUCTION:

Pressure measurement such as Gauge / Differential / Absolute pressure, flow in combination with primary element (orifice, venturi etc.) and level are measured by various types of electronic transmitters using different sensor characteristics and methods.

 Broadly transmitter has two parts – The sensor and the electronics.

The sensor consists of the pressure sensing device and the flanges / mating part to install it on the pipe line / equipment. The wetted parts are the oval flange, process flange, O-ring and the sensor diaphragm.

The electronics has IP65 housing with two numbers of cable gland entries. It also has a local indicator and zero, span buttons.

A well designed transmitter has separate compartment for electronics and cable termination. So that frequent opening of termination side does not affect electronics

OPERATING PRINCIPLE:

The most commonly used sensor is the capacitive sensor.

The isolating diaphragm detects and transmits the process pressure to the oil fill fluid. The fluid, in turn, transmits the process pressure to a sensing diaphragm located in the center of the cell. The reference pressure is transmitted in like manner to the other side of the sensing diaphragm. The sensing diaphragm deflects in response to differential pressure across it. The displacement of the sensing diaphragm, a maximum deflection of 0.004 in. (0.10 mm), is proportional to the applied pressure. The capacitance plates on both sides of the sensing diaphragm detect the position of the sensing diaphragm. The transmitter then converts this to a 4-20 mA signal.

TYPES OF SENSORS:

Classification based on operating principles:

There are different ways in which we can measure pressure. Each type has its own advantages and disadvantages.

Strain gauge is small, inexpensive device with good speed of response but is affected by process temperature variations.

Capacitive cell is manufactured by most of the vendors because of good accuracy, rangeability, and speed of response. They are temperature sensitive and have a low over-pressure capability but these shortcomings have been improved by continuous improvements.

Potentiometric sensor is low cost, small design version but involves mechanical wear.

Resonant wire has good repeatability, high resolution, strong output signal and generates an inherent digital signal which can be directly sent to a stable crystal clock in a microprocessor. The disadvantages are it has a non-linear output signal and is sensitive to ambient temperature variation and vibration.

Piezoelectric crystal has rugged construction, high speed of response but is sensitive to temperature variations.

Magnetic sensors: There are two types: a) Inductance b) Reluctance

Both the sensors have high output signal (40mV/V excitation), low sensitivity to shock and vibration due to rugged construction. But they involve moving parts.

Optical sensors are insensitive to temperature variation but dirt builds on the optics or aging of light source (LED) are the common problems.

Classification based on flange designs:

Conventional design: the inputs to the sensor capsule are given from two different planes of the sensor. These two sides could be exposed to different environmental conditions, probably one side of the sensor is always exposed to sunlight or hot environment (steam line or boiler operation). This could result in two sides of a sensor being exposed to two different temperatures. Moreover, the sensor is sandwiched between two flanges with O-ring in between as a result of which temperature gradient will not be uniform in two flanges. Some small temperature errors in measurement are found.

Co-planar design: the sensor is raised to the neck of the transmitter and there are two small capillary ports brought down to the isolating diaphragm and both input ports are in single flange. By this design, the two inputs are brought to the same plane and sensor is isolated thermally and mechanically away from the process. This results in better process temperature rating. (Previously it was 104°C, now it is 121°C).

SPECIFICATION OF SENSORS:

Some of the important specifications are listed below:

Whatever be the type of sensor, a diaphragm is required to transmit the pressure to the cell. The diaphragm is the main element, which isolates the sensor from the process fluids, at the same time transmitting the pressure to the sensor. Hence, care has to be taken in specifying the maximum pressure it has to withstand called as the over-pressure.

Compensation: Assume that a line ‘A’ having a static pressure of 200 bar is having a DP transmitter to measure flow with a range of 0-2500 mmWC. Also, assume line ‘B’ having a static pressure of 1 bar has a DP transmitter to measure flow with a range of 0-2500 mmWC. For these two cases, the accuracy will not remain same if the transmitters are calibrated at standard laboratory conditions.

Transmitters are calibrated in a laboratory at ambient temperature 25° C and at atmospheric pressures. In use, a transmitter is subjected to higher temperature and pressure. This will shift the zero and span. Error due to higher operating pressure is corrected by adjusting zero under operating pressure as a major shift is due to zero shifts. However, for higher pressures of 100 bar or above, some vendors take care of error during factory calibration. This compensation shall be specified only if operating pressures are more than 100 bar.

Process connection: Transmitters have ¼” tapped holes on the process diaphragms which hold the sensor. Three valve manifold shall be specified for all DP transmitters except for level measurement. When three valve manifold is specified, oval flanges are not required. Oval flanges are basically adapter flanges which will provide ½” NPT connection.

Fill fluid: Oils form an explosive mixture with strong oxidizing agents like oxygen and chlorine so fill fluid has to inert in case of oxygen and chlorine services.

Cleanliness: When a transmitter is made for oxygen service, oils shall not be used for calibration as it may cause an explosion. This shall be indicated in the specifications as a special requirement.

TRANSMITTER ELECTRONICS:

The extensive use of integrated circuits (IC) and surface-mount technology allows the electronics to be placed on a single circuit board. The transmitter’s microprocessor controls the operation of the A/D and D/A converters, performs diagnostic routines and facilitates digital communications.

During operation, the microprocessor processes a digital value and stores it as a digital word to enable precise corrections and engineering unit conversion. Next, the microprocessor performs sensor linearization (using values entered during the characterization procedure), temperature correction, damping, and other transfer functions to determine the correct value of input pressure.

During the characterization process at the factory, all sensors are run through pressure and temperature cycles over the entire operating range. Data from those cycles is used to generate correction coefficients that are stored in the sensor module memory to ensure precise signal correction during operation.

The EERPOM stores all configuration, characterization that can be changed by the transmitter software. This memory is non-volatile, and therefore, the data remains intact, even when no power is applied.

The microprocessor then sends the corrected digital word to the D/A converter for conversion to a standard 4-20 mA output signal. Also, at the same time, the mP drives simultaneous communications that provide a digital representation of the process variable.

Local span and zero push buttons provide convenient re-ranging at the transmitter site. Generally, in each transmitter, square root or linear output is software selectable. Communication is also accomplished with the help of hand-held communicator (HHT).

EVALUATION OF TRANSMITTERS:

MOC of wetted parts: The moc of process flanges, oval flanges and manifolds shall be as per pipeline / vessel moc. Minimum standard material for measuring element is SS316L. Other standard materials depending on process fluid are Tantalum, Hastelloy-C, Monel, Nickel.

Gasket: PTFE is suitable for all services. Many of the vendors offer viton as standard. Viton is not suitable for any chemicals such as ammonia, acetic acid, methanol, potassium hydroxide, sodium carbonate. Hence before accepting the vendor’s standard gasket, check with PE whether the same can be used for fluids under consideration.

MOC of external parts: Wherever ammonia is present even in small traces, exposed parts shall not be made of copper or copper-bearing alloys. Ammonia attacks Cu and its alloys.

Zero adjustment: Zero suppression and elevation facility is required for level transmitters.   When a transmitter is located below process nozzle (high pressure - HP), zero suppression is required. When a low-pressure (LP) side has liquid (compensation leg) has liquid, then zero elevation is required. Zero suppression and elevation must be such that neither the span nor the upper or lower range value exceeds 100% of the upper range limit

REMOTE SEAL SYSTEM

Remote seals are used when it becomes necessary to isolate the transmitter from the process. Such need arises under the following conditions:

• The process temperature specified is outside the maximum allowable operating temperature of the pressure transmitter. In any cases, this problem can be solved using the impulse piping, but it may not be possible always, and calls for a remote seal.
• The process is corrosive and would require frequent transmitter replacement or exotic material of construction.
• The process contains a lot of solids or viscous constituents that could plug the small ½ inch process connections of the standard pressure transmitter.
• The process demands process connections suitable and certified for use in sanitary applications.
• The process medium may freeze or solidify in the transmitter or the impulse tube.

EVOLUTION OF ELECTRONICS:

·         The electronics converts the pressure signal into a 4-20 mA signal, which is processed by controller/DCS to perform the further desired monitoring function. With the concept of controlling the complete process plant we need to establish communication with the field devices, else every time we have to go to field to re-range or recalibrate the instrument we have to go to field. This becomes difficult in hazardous area.

·         In order to upgrade the conventional transmitter a retrofit kit was offered. Smart retrofit allows remote communications, re-ranging and self-diagnostics by just removing the analog electronic boards and plugging in a new, single board smart electronic circuit having a microprocessor. There is also an E²PROM which stores the data. This data is retained in the transmitter when power is interrupted. The characteristics of each diaphragm are different due to metallurgy. But the vendor claims that over a period they know how each sensor behaves and so these characteristics are fed in the E²PROM. Due to this the same accuracy is achieved for a wider range i.e. Turndown has increased. Also, the concept of hand held communicator could be used with retrofit kit, which is extremely useful at site. Hand held communicators (HHT) are intrinsically safe (generally) and have re-chargeable batteries. But while making specifications the intrinsic certification has to be asked for.

·         The actual SMART transmitter has electronics module even in the sensor and the pressure gets converted into digital form incorporating the temperature correction. This sensor module also speeds repair. Because all of the module characteristics are stored with the module, the electronics can be replaced without having to recalibrate or remove separate correction PROMs.  The electronics module does further processing on this signal. In short the sensor became smart and the electronic module which was communicating between sensor and DCS became smarter. If exaggerated a bit we can say that the electronics in the transmitter is comparable with a single loop controller. The problem why we cannot use smart transmitter, as controller is any failure in the transmitter would collapse the complete control, which in case of remote PID controllers can be taken to manual mode. Further PID tuning needs expertise, if it can be tuned using hand held unit security is lost. Also, for proper tuning the SP, PV, TD, MV are required to be viewed simultaneously which is not possible in HHT as only one variable can be seen at a time.

·         There is a new concept called as the Hart maintenance System (HMS) where in all the information about the transmitters is stored and maintained. The output from transmitter is given to a patch card offered by DCS vendor where in the signal is split into 4-20 mA and digital signals. The digital signal is taken to HMS and 4-20 mA is taken to DCS. The software available with HMS is generally ‘Wonderware’ or ‘Cornerstone’. Problems may arise if smart transmitter is not compatible with the software version.

·         Definition of Smart transmitters – There is no “industry standard” definition, but certainly a truly smart transmitter should have the following characteristics and capabilities:

1) Microprocessor -based with predominantly digital electronics.

2) Remote communication and configuration

3) The ability to continuously monitor its operating conditions and correct itself for potential errors such as non-linearity, ambient temperature influence, static pressure influence, etc.

4) Continuous diagnostics of its sensing element and electronics as well as the loop power supply and wiring.

5) Transmitter can be programmed for high or low output (3.9 mA or 21 mA) in case of transmitter failure.

6) In most of the cases inventory is reduced because the transmitter PCB is common for all the transmitters (THL is an exception).