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Pressure Measurement

MADE BY:- Uttam Trasadiyatrasadiyauttam@gmail.com

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Pressure Measurement

• Pressure is the force exerted per unit area• Pressure is the action of one force against another force. Pressure is force

applied to, or distributed over, a surface. The pressure P of a force F distributed over an area A is defined as P = F/A

TOTAL VACUUM - 0 PSIA

PRESSURE

ABSOLUTE

GAUGECOMPOUND

BAROMETRIC RANGE

ATMOSPHERIC PRESSURENOM. 14.7 PSIA

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Pressure Measurement Terms

• Absolute PressureMeasured above total vacuum or zero absolute. Zero absolute represents total lack of pressure.

• Atmospheric PressureThe pressure exerted by the earth’s atmosphere. Atmospheric pressure at sea level is 14.696 psia. The value of atmospheric pressure decreases with increasing altitude.

• Barometric PressureSame as atmospheric pressure.

• Gauge Pressure The pressure above atmospheric pressure. Represents positive difference between measured pressure and existing atmospheric pressure. Can be converted to absolute by adding actual atmospheric pressure value.

• Differential Pressure The difference in magnitude between some pressure value and some reference

pressure. In a sense, absolute pressure could be considered as a differential pressure with total vacuum or zero absolute as the reference. Likewise, gauge pressure (defined above) could be considered as Differential Pressure with atmospheric pressure as the reference.

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Pressure Measurement

Pressure Units

psi 100

bar 6.895

mbar 6895

mm of Hg 5171

mm of WC 70358

in of WC 2770

Kg/cm2 7.032

Pascal 689476

kPa 689.5

atm 6.805

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Types of Pressure Instruments Pressure Gauges (Vacuum, Compound, Absolute, Gauge)Differential Pressure GaugePressure Switch (Vacuum, Absolute, Gauge)Differential Pressure SwitchPressure Transmitter (Vacuum, Absolute, Gauge)Differential Pressure Transmitter

Pressure Instruments

PRESSURE GAUGE

PRESSURE SWITCH

DIFFERENTIALPRESSURE

TRANSMITTER

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Pressure Gauge

PRESSURE GAUGES:• A Pressure Gauge is used for measuring the

pressure of a gas or liquid. • A Vacuum Gauge is used to measure the

pressure in a vacuum.• A Compound Gauge is used for measuring

both Vacuum and Pressure. • Pressure Gauges are used for Indication only.

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Pressure Gauge

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Measuring PrincipleBourdon tube measuring element is made of a thin-walled C-shape tube or spirally wound helical or coiled tube. When pressure is applied to the measuring system through the pressure port (socket), the pressure causes the Bourdon tube to straighten itself, thus causing the tip to move. The motion of the tip is transmitted via the link to the movement which converts the linear motion of the bourdon tube to a rotational motion that in turn causes the pointer to indicate the measured pressure.

Coiled Bourdon

Helical Bourdon

Pressure Gauge

“C” Type Bourdon

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Differential Pressure Gauge

Measuring Principle:• Differential pressure gauges have

two inlet ports, each connected to one of the volumes whose pressure is to be monitored.

• In cases where either input can be higher or lower than the other, a bi-directional differential range should be used.

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Differential Pressure Gauge

Unidirectional and Bidirectional DP Gauges

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Pressure Gauge - Accessories

Pulsation Damper (Adjustable Snubber)Threads on to a gauge socket and provides a restriction by means of a pin, which may be placed in either of five different sized holes, and thus allows the user to vary the amount of dampening to suit requirements. The pulsating pressure moves the pin up and down, providing self cleaning action.

Safety Glass FrontSafety Glass is normally used to prevent the glass shattering in the event of the bourdon tube rupturing.

Liquid Filled GaugeThe liquid filling is used to dampen any vibration/pulsation in the bourdon, either silicone oil or glycerin is used.

SnubberUsed for dampening and filtering and reducing the damaging effects of pulsation on a gauge. The snubber has a metal disc available in standard grades of porosity.

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Pressure Gauge - Accessories

COIL PIPEPIG TAIL

Pressure Limit Valve Protects pressure instruments against surges and pulsations. Provides automatic positive protection and accurate, repeatable performance. Automatic pressure shut-off, built in snubber enhances instrument protecting performance.

Siphon TubesUsed to dissipate heat by trapping condensed liquid to keep high temperature steam or condensing vapor from damaging the pressure gauge.

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Diaphragm Seals

Diaphragm seals, also known as chemical seals, isolate pressure measuring instruments from the process media. The system pressure is transmitted to a fill fluid in the upper housing of the diaphragm seal, and from there to the pressure-measuring instrument itself. The use of diaphragm seals should be discussed with and approved by the Client.

Diaphragm sealed gauges should be considered for:• Process fluids that would clog the pressure elements.• Process fluids that are toxic, corrosive, slurried and viscous.• Process fluids that could crystallize or polymerize.• Materials capable of withstanding the process fluids that are not available

as a pressure element, such as high temperature.• Process fluid that might freeze due to change in ambient temperature and

damage the element.• Hydrocarbon services having a Reid vapor pressure (RVP) of 18 psig and

over. (RVP is the absolute vapor pressure exerted by a liquid at 100°F. The higher this value, the more volatile the sample and the more readily it will evaporate).

• Auto-ignitable hydrocarbon services.

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Diaphragm Seals

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When selecting a Pressure Gauge, care should be given to a number of parameters which have an effect on the gauge’s accuracy, safety, and cost.

• Accuracy required • Dial size • Operating pressure range • Chemical compatibility with gauge construction

materials • Operating temperature range • Vibration, pulsation, and shock • Pressure fluid composition • Mounting requirement

Pressure Gauge Selection Guideline

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Pressure Gauge Ranges

• Since the accuracy of most pressure gauges is better in the middle portion of a gauge, you should always select a gauge with a range that is about double your normal anticipated pressure.

• The maximum operating pressure should not exceed 80% of the full pressure range of the gauge.

• Standard pressure ranges are measured in PSI, Bar, Pa or kPa and most of the gauges have dual PSI/metric scales.

• Very low pressure gauges have scales that measure in Inches of H2O, mm H2O, and Inches of Hg. 

• Vacuum gauges have scales in inches of mercury, while compound gauges have scales that measure in both vacuum and pressure.

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Pressure Gauge Installation

Gas Service

Liquid Service

• Top connection preferred for gas installations & side connection preferred for liquid installations.

• The pressure gauge can be connected to the pipe by individual block and bleed valves or a two way manifold.

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Pressure Switch

Measuring Principle:

• The device contains a micro switch, connected to a mechanical lever and set pressure spring. The contacts get actuated when process pressure reaches the set pressure of the spring.

• It can be used for alarming or interlocking purposes, on actuation.

• It can be used for high / high-high or low / low-low actuation of pressure in the process . The set range can be adjusted within the switch range.

• The sensing element may be a Diaphragm or a piston

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Pressure Switch

• Pressure/Vacuum Switch - A device that senses a change in pressure/vacuum and opens or closes an electrical circuit when the set point is reached.

• Pressure switches serve to energize or de-energize electrical circuits as a function of whether the process pressure is normal or abnormal.

• The electric contacts can be configured as single pole double throw (SPDT), in which case the switch is provided with one normally closed (NC) and one normally open (NO) contact.

• Alternately, the switch can be configured as double pole double throw (DPDT), in which case two SPDT switches are furnished, each of which can operate a separate electric circuit.

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Pressure Switch

• The switch housings can meet any of the NEMA standards from Type 1 (General Purpose) to Type 7 (Explosion Proof), or Type 12 (Dust Proof) or Type 4 (Water Proof).

• Pressure switches are also available in hermetically sealed enclosures.

• Gold plated contacts are available for reliability .

• Pressure Switches are not as commonly used today, since they contain mechanical moving parts and moving parts are significantly more likely to fail than transmitters and the failures can go undetected.

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Pressure Transmitter

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Pressure Transmitter

• A Pressure Transmitter is used where indication and/or record of pressure is required at a location not adjacent to the primary element.

• A Pressure Transmitter is used for both indication and control of a process.

• A Pressure Transmitter is used where overall high performance is mandatory.

• Both Electronic and Pneumatic Transmitters are used.• These can be either Gauge, Absolute or Differential

Pressure Transmitters.

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Transmitter Measuring Principle

• The diagram shows an electronic differential pressure sensor. This particular type utilizes a two-wire capacitance technique.

• Another common measuring technique is a strain gauge.

• Process pressure is transmitted through isolating diaphragms and silicone oil fill fluid to a sensing diaphragm.

• The sensing diaphragm is a stretched spring element that deflects in response to the differential pressure across it.

• The displacement of the sensing diaphragm is proportional to the differential pressure.

• The position of the sensing diaphragm is detected by capacitor plates on both sides of the sensing diaphragm.

• The differential capacitance between the sensing diaphragm and the capacitor plates is converted electronically to a 4–20 mA or 1-5 VDC signal.

• For a gauge pressure transmitter, the low pressure side is referenced to atmospheric pressure.

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Pressure Transmitter

• Typical Outputs–4 to 20 milliamp (mA). analog signal–Smart HART digital signal (superimposed on

analog signal)–Fieldbus digital signal–3 to15 psi pneumatic signal

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Diaphragm Seal System

• A diaphragm seal system consists of a pressure transmitter, diaphragm seals, a fill fluid, and either a direct mount or capillary style connection.

• During operation, the thin, flexible diaphragm and fill fluid separate the pressure sensitive element of the transmitter from the process medium. The capillary tubing or direct mount flange connects the diaphragm to the transmitter.

• When process pressure is applied, the diaphragm transfers the measured pressure through the filled system and capillary tubing to the transmitter element.

• This transferred pressure displaces the sensing diaphragm in the pressure-sensitive element of the transmitter.

• The displacement is proportional to the process pressure and is electronically converted to an appropriate current, voltage, or digital HART output signal.

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Diaphragm Seal System

•WHY USE DIAPHRAGM SEALS?

Diaphragm Seal systems provide a reliable process pressure measurement and prevent the process medium from contacting the transmitter diaphragm.

Transmitter/ Diaphragm Seal systems shall be used for:• For process fluid that would clog the pressure elements.• For process fluids that are toxic, corrosive, slurry and viscous.• For process fluids that could freeze or solidify.• For process temperatures outside the normal operating range and cannot

be brought to those limits by impulse piping.• For process that needs frequent cleaning.• For processes that need replacement of wet legs, to reduce

maintenance.

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When selecting a pressure transmitter care should be given to a number of parameters which have an effect on transmitter accuracy, safety, and utility.

• Accuracy required • Power supply • Operating pressure range • Operating temperature range • Body Material • Pressure fluid composition and Internal parts • Mounting requirement• Process connection size

Pressure Transmitter Selection Guideline

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Pressure Transmitter installation

• Mounting above tap is typical for gas service and mounting below tap typical for liquid and steam services.

• Direct mount is possible for low temperature services.

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Diaphragm Seal installation

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Pressure Instruments

Selection of Pressure Instruments: Rules of Thumb:• Application: Understand your application. Examine the

particulars of your application. Is it necessary to know if the pressure is negative or positive? Do you need to know the difference in pressure between two points? Answering these questions about your application will go a long way in helping select the right pressure transmitter.

• Wetted Parts: Selecting the transmitter with wetted parts that are compatible with the medium to be measured helps to ensure a long-lasting measurement solution.

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Pressure Instruments

• Accuracy: From an accuracy point of view, the range of a transmitter should be low (normal operating pressure at around the middle of the range), so that error, usually a percentage of full scale, is minimized. On the other hand, one must always consider the consequences of overpressure damage due to operating errors, faulty design, or failure to isolate the instrument during pressure-testing and start-up. Therefore, it is important to specify not only the required range, but also the amount of overpressure protection needed.

• Output Required: Pressure transmitters can send the process pressure of interest using an analog pneumatic (3-15 psig), analog electronic (4-20 mA dc), or digital electronic signal.

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Pressure Instruments

• Protection: Do you need special protection from the elements? Many applications require special protection, such as, corrosive environment, or an outdoor environment. Pressure transmitters are available in various NEMA ratings or can be assembled in special NEMA rated housings that help protect them from harsh environments.