Pressure Measurement

PRESSURE

The entire pressure measurement instruments in the refinery can be very broadly classified into following categories.

• Pressure Transmitters
• Pressure gauges
• Pressure switches
• Pressure calibrators

Pressure Transmitters

The pressure transmitters are used to measure the pressure and transmit the reading in electrical / electronic signals for remote indication. They can be classified into three categories namely Absolute, Gage and Differential transmitters.

The absolute instruments measure the pressure with respect to absolute zero (perfect vacuum) and are generally used in vacuum column pressure measurement.

The gage pressure is the pressure with respect to the local atmospheric pressure and they are mainly used for all pressure measurement.

The differential pressure transmitters measure the pressure difference between two points and are the most widely used instrument in the refinery, when compared to any other type of process instrument. Almost all the restriction type of flow sensors uses the differential pressure transmitter to convert the generated differential pressure to an electronic signal. A large number of level measurement instruments using the hydrostatic principle also used the differential pressure transmitter. They are also used for purge type level measurement.

The common make of these transmitters are Rosemount, Yokogawa, Honeywell, Fuji, Siemens, ABB, Yamatake etc. Different vendors use different sensing technologies as per the type, range & accuracy required and the common technologies used are variable capacitive, variable reluctance, piezo electric, semiconductor etc. The transmitter output earlier were analog but recently digital transmitters (SMART) are also used (mainly using HART protocol). In case of smart transmitters, the communication to the control rooms are still done using analog signal (4-20 mA) but with the help of hand held communicators and PCs loaded with suitable communication software digital communication to the transmitter is also utilized. This digital communication enables easy re ranging and easy problem diagnostics. Moreover smart transmitters give very wide range ability.

In case of liquids, which may get concealed in pipes (impulse) due to temperature, it is better to provide stream tracing (1/4” OD Copper tube) wound round the impulse piping (coiled) up to the instrument. After tracing it, cover the exposed tubing with asbestos rope and apply “thermo cement” which will help to retain the temperature to avoid plugging. The effectiveness of the steam tracing depends on the way it is wound and also the proper working of the steam traps.

Pressure gauges

Pressure gages are used to measure and locally indicate the pressure. They are very cheap when compared to pressure transmitters but are less accurate and only local dial indication is possible.

A pressure gage consists of socket, sensing element, tip movement, case, die and pointer, lens, bezel, optional feature and accessories. The figure shows the basic items:

The socket connects the pressure source to the sensing element and also it supports the other components. Connection sixe is generally ½ “ NPTM. Other connections like ¼”, 1/8 “ and BSP threads are also used. Socket is made of brass, stainless steel and are made from bar stocks.

The sensing elements used are mainly Bourdon, bellows and diaphragm type. The tip of the pressure gauge acts as a seal and also to provide attachment to the pointer. The movement consists of geared section and pinion. Heleical, magnetic and gear less. The case encloses all the internal parts. The mainly used accessories are pulsation damper, chemical seals and siphons.

Sensing elements:

a) Bourdon tube

Bourdon tube is the most commonly used pressure sensor and are known as deformation elements. There are three types of bourdon available namely C – Bourdon, spiral and helical.

Advantages

1. Low cost
2. Simple construction
3. High pressure application

Limitations

1. Precision measurement only below 60 psi
2. Susceptible to shock and vibration
3. Subjected to hysteresis

The bourdon tube is secured at one end in a fixed socket where the sensing pressure is applied and the other end is sealed. With the pressure applied, the tubes straighten. This movement is balanced against the spring of the tube. The movement of the tube is non linear but linearity is obtained through a geared pinion and sector movement.

Spiral bourdon tube is used where the free end of C type is not enough to provide the requisite motion. Since greater movement of the free end is attained with the spiral element, better accuracy is obtained.

Helical type bourdon is similar to spiral except it is wound in the form of helical. The number of coils depends on the range of the instrument. These gauges provide more power than spiral elements and have over range capabilities.

The common materials used are phosphor bronze (low pressures and non corrosive services), Beryllium copper and SS 316 (high pressure and corrosive).

b) Bellows:

Bellows are usually formed from a thin seamless tube. In order to get better accuracy and long life, its movement is supported by spring so that only a part of the maximum stroke is used. This system is termed as spring-loaded bellows. They are used to measure absolute gauge and differential pressure. While measuring absolute pressures, two bellows are to be used. The evacuated bellow is used for atmospheric pressure compensation. The greatest use of bellows elements probably is that of receiver elements, for receivers and controllers for pneumatic instrument, also in low range pressure switches. Usually they are made of brass, phosphor bronze, beryllium copper, SS or monel. It is widely used in low pressure measurement.

c) Diaphragm:

The operating principle is similar to that of the bellows. Pressure is applied to the element, causing it to expand in direct proportion to the pressure applied. The movement of the diaphragm depends on the thickness and diameter of the material used. Diaphragm element is either a flat disc or a corrugated surface. In some cases two diaphragms are bonded together at their circumstances and are termed as capsule. When measuring absolute pressure, the capsule has to be evacuated. Materials of construction available are: Beryllium copper, Ni-Span C, Inconel, Monel, Hastalley & Nickel. Used mainly in low ranges.

Accessories:

The mainly used are:

(a) Pulsation dampness
(b) Chemical sears
(c) Pigtail siphons

a) Pulsation damper:

Pulsation dampers or snubbers are needed when pressure cycling is periodic enough to damage the measuring sensors. A damper is a restriction device, which produces a sufficient resistance to flow that pressure at the element changes slowly enough for the element to respond.

b) Chemical seals:

When the process liquid is corrosive, to prevent the bourdon from damage, a seal is used. It can also be used where the process liquid may clog the leads. A seal is constructed of thin flexible diaphragm as a separating member between the process fluid and the pressure element. The space above the diaphragm and all connections to the element are filled with a selected temperature stable liquid. The movement of the diaphragm is transmitted through this liquid system. This type of instrument operates satisfactorily in all applications. But the limitation is the volumetric capacity of the seal. This is kept to a minimum but sufficient enough to move the pointer to its full scale.


c) Pigtail siphons:

They are used in (i) to form a liquid seal in condensate vapour services (ii) to isolate hot vapours from reaching the sensing element. It is widely used in steam services.


Pressure switches

Pressure switches are widely used in a variety of process applications to give a contact output (open or close i.e., NO or NC) when the sensing pressure reaches a particular value called the set value. They can be high or low switch. A high switch, switches the contact on the rising pressure and low switch generates the contact on the lowering pressure. These contacts will be suitably wired to other control systems for necessary actions (indication or control).
The pressure to be sensed is applied to a sensor (manometric, tube, bellows, bulb). Whenever there is a change of pressure, the sensor is distorted and acts on a force balance connected to a microswitch.
Opposite this sensitive element, an adjustable spring enables the working point to be adjusted: whenever the pressure on the sensor is sufficient to counter the spring pressure, the force balance changes position.
Adding a second adjustable spring permits greater or lesser increase of the hysteresis (dead-band) or to act on dislocation between the two contacts if the instrument is thus equipped.
Sensors:
a) Bellows: The most conventional sensitive element used in our pressure and temperature switches is a metallic bellows (bronze or stainless steel) made up of a corrugated tube fitted with two airtight rigid ends. Flexible by nature, it compresses or elongates axially under the effect of pressure and provides excellent transmission.

b) Diaphragm: For low pressure, bellows, which are too rigid, are replaced by an elastomer diaphragm, which is hold in place, by two metal plates. "Diaphragm" technology is also used in cases of excessively high pressure and pulsating phenomena.
c) Manometric tube: For very high pressures, the sensor used is a stainless steel tube curved into a C shape which is closed at one end and which changes position under the effect of pressure.
Differential pressure switch
A differential pressure switch is fitted with two connections: one of them (HP) is connected to the highest pressure, the other (BP) to the lowest pressure which means, in the majority of cases, that it has to be fitted the right way round. These two connections correspond either to two sets of bellows fitted opposite one another or to the two sides of the diaphragm. The resultant of the two different pressures acts on the internal mechanism, which then functions like a standard instrument.
Absolute pressure switch
The pressure switch is fitted with a differential measuring element with bellows, one of which is vacuum-sealed so as not to be affected by atmospheric pressure.

Pressure Calibrators

Pressure calibrators are used to calibrate the pressure measuring instruments. They are kept in the instrument lab.

Portable pressure calibrators:

Common among them are portable pressure calibrators, which can be taken to field and calibrate the instruments. These portable calibrators have a hand pump and accurate pressure indicating instrument (digital or analog) for comparing the reading. These calibrators are available in different ranges. The common make are Scandura, Druck, Nagman, Ametek, Beamex etc.

Apart from these portable devices manometers and dead weight testers are also used for calibration.

Manometers:

Manometers are used for calibrating low range instruments in lab. Their advantage is that they are very simple to construct, high accuracy and good repeatability. Sometimes they are also used as measuring instruments in the field also for low-pressure purpose. They are classified as gravity balance pressure device. They measure unknown pressures by balancing against the gravitational force of liquid heads. A wide variety of liquids are used as manometer fluid. Their specific gravity varies from 1 to 13.6. The main criteria for the fluid are non-corrosive, stable, non-toxic and compatible with the measured fluid in contact with the manometer liquid. The advantages of manometers are simple and time proven construction

Different types of manometers are
a) U-type
b) Well type
c) Inclined manometer

a) U-type

This is widely used as a differential pressure-measuring device. With one end open to atmospheric, it can be used to measure pressure.

b) Well type

This is similar to U type except for a reservoir on the high-pressure side.

c) Inclined manometers

It provides a greater accuracy in low range of measurements. A small change in vertical height provides an amplified liquid movement along with slopes. Available ranges are 0 - 0.5 inches of H2O and 0 – 4 inches of H2O.

Dead weight Tester

This is used as a primary standard to calibrate other pressure elements. When used with the controlled pressure source, it is termed as “Dead Weight Piston Gauge”. In principle, a piston of known area is inserted into a close fitting cylinder. Weights loaded on one side or the piston are balanced by fluid pressure applied at the other end. The piston areas are precision built and the weights are certified.

The tester available in our shop is provided with two pistons, one for lower ranges and the other for higher ranges. All calibration of pressure gauges and pressure transmitters are done using this weight tester in the shop.