Pressure measurement

PRESSURE MEASUREMENT

Pressure is an important factor should be measured in all industries.

Force per unit area is called pressure (P).and it is like a stress, but stress is a term used only in

solids, whereas pressure is used in fluids (liquid, gas and steam applications).

Units of pressure:

SI standard: N/m2 (Pa-Pascal), Kpa (Kilo Pascal)

Other units: 1mmH2O= 9.80665 pa

1mmHg= 133.32239 pa

1Bar=100kpa

1atm=101.325Kpa

CGS standard: Kg/cm2

1kg/cm2= 98.0665 Kpa

US/UK standard: Pounds per square inch- psi (lbs. /in2)

1psi=6.89475728 Kpa

Atmospheric pressure:

Sea level standard atmospheric pressure is 101.325Kpa but it changes with altitude of earth.

1 atmosphere [technical] is equal to 98.0665Kpa.

Absolute pressure:

Absolute pressure is zero-referenced against a perfect vacuum, so it is equal to gauge pressure

plus atmospheric pressure.

‘a’ letter is appended if pressure is measured in zero reference Ex: psia, bara etc…

Gauge pressure:

Gauge pressure is referenced to atmospheric pressure.

Gauge pressure=absolute pressure-atmospheric pressure.

‘g’ letter is appended if pressure is measured in gauge pressure Ex: psig, barg etc….

Differential pressure:

Differential pressure is difference between unknown pressure and known pressure (constant).

It is like gauge pressure but instead of atmospheric pressure (changing with altitude) as a

reference some known (constant) pressure is created and differential pressure is measured.

‘d’ is appended if pressure is measured as differential pressure Ex: psid, bard etc…

Pressure Gauges:

Pressure gauges are instruments used to measure the gauge pressure.

There are mainly three elements used in pressure gauges

1) Bourden tube

2) Diaphragm

3) Bellows

Depend on the application (process conditions) and pressure range we choose the pressure

elements.

Bourden tube pressure gauge

Typical pressure gauge is shown below,

Working:

Bourdon tube is elastic transducer which is fixed at one end and open at the other end to

receive the pressure which is to be measured. The other end of the bourdon tube is free and

Closed. The cross-section of the bourdon tube is elliptical. The bourdon tube is in a bent form

to look like a circular arc. To the free end of the bourdon tube is attached an adjustable link,

which is in turn connected to a sector and pinion as shown in diagram. To the shaft of the

pinion is connected a pointer which sweeps over a pressure calibrated scale.

The pressure gauge is calibrated such that it has to show zero on the dial against the

atmospheric pressure it means at rest condition. As pressure increases, the displacement of the

bourden tube increases and movement is mechanically adjusted for proper displacement of

pointer .once mechanical adjustment is perfect, it will indicate proper output on the dial which

has linear scale.

Diaphragm pressure gauge

Diaphragm is elastic material which displaces, when it is applied to pressure.

As the fig. shown above diaphragm is a metallic elastic material which deforms when pressure

increases and it is connected to mechanical linkage and movement is transferred to sector and

pinion and by mechanical movement that is indicated on dial by pointer.

Bellows pressure gauge

Bellows elastic material uses the same principle of working but against the spring load.

As pressure increases the compression of bellows, spring takes place and decrease in pressure

made spring to elongate and this movement is transferred to sector, it will turn the pinion gear

and same is indicated on dial.

Accessories of pressure gauge

Fluid filled pressure gauge:

Pressure gauge be filled with fluid inside to avoid the movement of pointer due to mechanical

vibration and also used as temperature inhibitor.

Snubber:

If process has pressure spikes, pulsations and hammering of fluid that is developed by pump or

process itself may damage the pressure gauges so we need to protect the pressure gauge from

that surges. This can be done using the snubbers.

While the Snubber does not alter the pressure of the process, it will lessen the pulsation on the

line, thereby improves the gauge reading, and which extends the lifetime of the pressure

gauge.

Generally snubbers used above 5psi.

There are mainly 3 types of snubbers

1) Porous Snubber

2) Piston Snubber

3) Adjustable Snubber

Fig. Porous type Fig. Piston type Fig. Adjustable type

Syphon:

Syphons are used where high temperature vapors and fluids which damage the pressure gauge

by cooling or by dissipating the heat.

There are mainly 3 types of syphon

1) Coil type

2) U tube type

3) Pig tail type

Fig. coil type Fig. u tube type Fig. pig tail type

U tube type are mainly used in vertical pipes, and whereas coil type used mainly in horizontal

pipe.

Pressure limiting valves:

Pressure limiting valves are also called gauge savers or overload protectors.

Pressure limiting valve shuts off the pressure gauge service, when there is a high pressure

reached over the desired pressure (pre-set value) by using the push rod and again puts in

service when pressure decreases the desired value. That desired value is set by spring which is

attached to push rod.

The desired pressure (pre-set value) can be adjusted using adjust screw.

Blowout disc

Blow out discs are used in releasing the pressure when of the gauge inside pressure increases

due rupture or leakage in the bourden tube.

There are mainly two types of blow out discs rear or back blowout disc and solid front blow out

disc.

Fig. Back or rear blow out disc

Solid front blowout disc uses solid front between the pointer, dial, window and lens in one side

and mechanical parts, bourden tube on the other side. So it will not affect the lens, pointer and

dial.

Pressure gauge ring (Bezel)

Pressure gauge ring is used to connect the case and the window

There are mainly 4 types of ring types available

1) Screwed ring/ threaded ring

2) Hinged ring

3) Slip ring

4) Bayonet ring.

Pressure gauge lens material (window)

The lens material may be glass or plastic

If it is glass it should be shatter proof. If there is no breakage concern use glass type lens.

If there is a breakage concern use the plastic lens or shatter proof glass lens.

Dial

Dial size depends on the accuracy required.it means higher accuracy requires bigger size of dial.

Case

Case is main part in pressure gauge tube which encloses bourden tube, movement, pointer and

dial. There are many dial sizes available from 40mm to 250mm diameter. The material depends

on the surrounding environment i.e. if corrosive environment we use stainless steel (SS-316),

cast iron for more ruggedness and aluminum for less weight.

Type of mounting and connection position

There are mainly 3 mounting style of pressure gauge,

1) Direct/local mounting

2) Surface mounting

3) Flush or panel mounting

If you come to connection position 3 types are there as follows

1) Radial connection(bottom)

2) Center back connection

3) Offset back connection

Direct or local mounting

As name only suggests that, the pressure gauge is mounted in local i.e. near process.

Fig. direct mounted

We can use all three connection position for direct or local mounted pressure gauge.

Radial connection center back connection offset back connection

Surface mounting

Surface mounting uses wall or pipe to mount the pressure gauge by back flange. So in surface

mounting the process connection should be bottom only (center back and offset back

connection not preferred).

Radial connection center back connection offset back connection

Flush mounted

Flush mounted is also called panel mounted, so it uses front flange connection.

Flush mounted uses only back connection so bottom connection are not preferred.

Flush mounted can be done using clamp fixing or three holes on the flange and it is fitted on

panel box.

Radial connection centered back connection offset back connection

Selection of pressure gauges:

Pressure gauge selection depends on the pressure range, process fluid and temperature.

Pressure range

Bourden tube: 1.6 bar to 1600 bar

Diaphragm: 2.5 mbar to 25 bar

Capsule: 1mbar to 600mbar

Process fluid

Bourden tube can be used for all gas or stream applications and for low viscosity liquid

applications.

Special arrangement like diaphragm seal can be used for high viscosity and high corrosive

liquid applications.

Diaphragm can be used for all gas, stream and liquid applications.

Bellows are applicable very good for gas, stream applications compared to liquid applications

that to only low viscosity liquid and the condition is bellows and pipe should be completely

filled.

Pressure range selection:

The maximum working pressure should be less than 75% (2/3rd) of the maximum scale

(calibrated) for static pressure.

The maximum working pressure should be less than 65% (1/3rd) of the maximum scale for

cyclic pressure.

Accuracy:

Of course accuracy depends on dial size, but dial size depends on the case size.

Case sizes are vary from 40mm to 250mm.

The following accuracy classes are defined: 0.1, 0.25, 0.6, 1, 1.6, 2.5 and 4 based on the dial size. The below fig. shows accuracy class and it shows that If we use 40 or 50 mm case size (OD), we can expect accuracy up to 1.6%, 2.5%, and 4% of span If we use 250 mm case size, we can expect accuracy up to 0.1%, 0.25%, 0.6%, 1% and 1.6% of span.

Note: Refer Standard EN-837 pressure gauge types, selection and installation.

Diaphragm seals: Diaphragm seals are used in in applications where pressure gauge elements are not to be wetted with the process fluid which is more corrosive and high viscosity fluid. A diaphragm within the diaphragm seal separates the gauge from the process medium

Working: It mainly consist of diaphragm, gasket, flange, system fill fluid and capillary tube (if necessary) Only lower housing, diaphragm and gaskets are exposed to the process medium is selected from materials resistant to pressure, temperature and possible chemical attack by the process medium. The diaphragm seal is also filled with a transmitting fluid or system fill fluid. Any pressure applied by the process medium to the seal diaphragm is hydraulically transmitted to the pressure element of the gauge, thus generating a pressure reading.

Installation of pressure gauge

The user shall ensure that the correct gauge has been selected and has the correct range and construction. If necessary an isolating valve shall be inserted to facilitate removal for maintenance. Pressure connections shall be leak tight. Gauges with parallel threads: parallel threads are straight type and the pressure seal is made on the sealing face using a sealing washer which is compatible with the fluid. In general we go for ½ “ NPTM threaded connection.

Gauges with tapered threads: The pressure seal is normally made by the mating of the thread, but it is common practice to apply jointing material to the male thread before assembly. The jointing material shall be compatible with the fluid.

Pressure transmitter

Pressure transmitter is an instrument used for indicating, transmitting, controlling and

recording of pressure variable.

Pressure transmitter mainly consist of primary element which is connected to process for

pressure measurement (diaphragm, bellows or bourden tube) and secondary electronic

package which amplifies, linearize, and produce an equivalent 4-20mA current output to DCS or

any control system by 2 wire loop powered or 4 wired communication. Also we can use HART

analog/ digital (SMART) type communication for sophisticated transmission.

If diaphragm pressure element (up to 2.5mbar to 25bar) is used as a primary element then

conversion of pressure into electrical output by using either any one of the method like strain

gauge, variable capacitance, variable reluctance or Piezoresistive.

Then we go for amplification, linearization and if needed digitization for digital transmission by

field bus communication.

Display may be LCD or LED type for field indication.

Depends on the surrounding environment, it should have hermetically sealed enclosure Ex: IP

65

Based on hazardous area classification, explosion due to sparks, temperature and lightening

effects can be prevented by using methods like flame proof, intrinsically safe, increased safety

or non –incendive type of protection.

There are many mounting methods for transmitters like direct (local) mounted, yoke mounted,

flush mounted and surface mounted(either pipe or surface).

Pressure switches

A pressure switch is a form of switch that closes an electrical/pneumatic contact when a certain

set pressure has been reached on its input. The switch may be designed to make contact either

on pressure rise or on pressure fall.

There are mainly two types of pressure switches namely Electric pressure switch and

Pneumatic pressure switch.

A pressure switch for sensing fluid pressure contains a capsule, bellows, Bourdon tube,

diaphragm or piston element that deforms or displaces proportionally to the applied pressure.

The resulting motion is applied, either directly or through amplifying levers, to a set of switch

contacts. Since pressure may be changing slowly and contacts should operate quickly, some

kind of over-center mechanism such as a miniature snap-action switch is used to ensure quick

operation of the contacts.

One sensitive type of pressure switch uses mercury switches mounted on a Bourdon tube; the

shifting weight of the mercury provides a useful over-center characteristic.

Operation:

The contacts in a pressure switch may be normally closed when the pressure is below the set point for example shown in fig; the contact in a normally open switch remains opened until the pressure rises above the set point. Then the sensing element makes the contacts snap to close position. The contacts open again when the pressure falls below the set point. The contacts in a normally closed switch remain closed until the pressure rises above the set point. Then the contacts open and remain open until a pressure drops below the set point again. Most switches contain the two sets of contacts, one normally open and other normally closed. Choices for pressure switches measurement ranges include absolute, gauge, vacuum, differential, and sealed. Absolute switches are used where pressures are to be measured independently of the natural fluctuations in atmospheric pressure. Vacuum measurement switches measure vacuum pressure (negative pressure). Differential pressure switches give the relative pressure between two points. If both operating

pressures are the same, the measuring element cannot move and no pressure will be indicated.

A differential pressure is indicated when one pressure is higher or lower. Low differential

pressures can be measured directly in cases of high static pressures.

The above shown fig. is SPDT (Single Pole Double Through) pressure switch and even switch

may be DPDT (Double Pole Double through) switch depend on application.

The set point can be adjustable. And we have two differential

1) Fixed differential

2) Adjustable differential

In fixed differential (one set point )we can change cut in pressure (minimum pressure) and cut

off pressure (maximum pressure) by using screw (spring adjustment), but within the fixed

differential.

In adjustable differential (two set point) we can change cut in pressure (minimum pressure) and

cut off pressure (maximum pressure) by using extra screw for adjustable difference.

Usually we go for diaphragm and piston type elements.

Max. Allowable pressure for diaphragm is 850psi or less whereas piston type has 10,000psi

or more

Maximum differential we can develop from diaphragm is 20psi and 60psi or more from piston

type.

Diaphragm is more sensitive to compared piston.

Diaphragm mainly used for hydraulic applications and piston is used for pneumatic applications.

Depends on the surrounding environment, it should have hermetically sealed enclosure Ex:

IP 65

Based on hazardous area classification, explosion due to sparks, temperature and lightening

effects can be prevented by using methods like flame proof, intrinsically safe, increased safety

or non –incendive type of protection.

.