# Mark Murphy, PE Technical Director, Fluor Corp.

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Mark Murphy, PE Technical Director, Fluor Corp.
Pressure Measurement ISA = International Society of Automation Mark Murphy, PE Technical Director, Fluor Corp.

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 Pressure is defined as the force acting on a unit area.

Pressure Measurement Terms
Absolute Pressure Measured above total vacuum or zero absolute. Zero absolute represents total lack of pressure. Atmospheric Pressure The pressure exerted by the earth’s atmosphere. Atmospheric pressure at sea level is psia. The value of atmospheric pressure decreases with increasing altitude. Barometric Pressure Same 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. When distinguishing between these zero references, the following terms are used: Absolute pressure is zero referenced against a perfect vacuum, so it is equal to gauge pressure plus atmospheric pressure. Atmospheric Pressure is the pressure exerted by the earth’s atmosphere. Atmospheric pressure at sea level is psia. The value of atmospheric pressure decreases with increasing altitude.

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 These are the various PRESSURE UNITS used most frequently.

DIFFERENTIALPRESSURE TRANSMITTER
Pressure Instruments Types of Pressure Instruments Pressure Gauges (Vacuum, Compound, Absolute, Gauge) Differential Pressure Gauge Pressure Switch (Vacuum, Absolute, Gauge) Differential Pressure Switch Pressure Transmitter (Vacuum, Absolute, Gauge) Differential Pressure Transmitter DIFFERENTIALPRESSURE TRANSMITTER Many techniques have been developed for the measurement of pressure and vacuum. Instruments used to indicate pressure directly include pressure gauges, vacuum gauges or compound gauges. Differential pressures are commonly used in industrial process systems. Differential pressure gauges have two inlet ports, each connected to one of the volumes whose pressure is to be monitored. In effect, such a gauge performs the mathematical operation of subtraction through mechanical means, eliminating the need for an operator or control system to watch two separate gauges and determine the difference in readings. Pressure and differential pressure switches are supplied with a relay or mechanical switch which changes state as the pressure rises above or falls below a preset pressure. Pressure transmitters are used for absolute, gauge or differential pressure to send an electronic analog or digital signal to a remote location to control or indicate the pressure. PRESSURE SWITCH PRESSURE GAUGE

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. This defines the use of various pressure gauges.

Pressure Gauge Many techniques have been developed for the measurement of pressure and vacuum. The simplest of these is the bourdon tube pressure gauge.

Pressure Gauge “C” Type Bourdon Measuring Principle
Bourdon 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. The Bourdon 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 or socket, the pressure causes the Bourdon tube to straighten itself, thus causing the tip opposite the socket to move. The motion of the tip is transmitted via the link to the movement which converts the motion of the bourdon tube to a rotational motion that in turn causes the pointer to indicate the measured pressure. There are three commonly used measuring elements in Pressure gauges they are the C-Type Bourdon, the Helical Bourdon and the Coiled Bourdon. Helical Bourdon Coiled Bourdon “C” Type Bourdon

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. Differential pressure gauges have two inlet ports, each connected to one of the volumes whose pressure is to be monitored. In effect, such a gauge performs the mathematical operation of subtraction through mechanical means, eliminating the need for an operator or control system to watch two separate gauges and determine the difference in readings. The gauge shown here utilizes two bourdon tubes one for each pressure measurement and a linkage to determine the differential pressure. A diaphragm or bellows can also be used as the pressure sensing element. In cases where either input can be higher or lower than the other, a bi-directional differential range should be used. Bidirectional Differential Pressure gauges are constructed such that the zero point is in the middle. The opposing Bourdon tubes are linked to a single pinion gear which rotates a pointer for direct pressure readings. By using two independent Bourdon tubes, the gauge can handle liquids or gases on either or both ports.

Differential Pressure Gauge
Unidirectional and Bidirectional DP Gauges The gauge shown on the left is a bidirectional gauge and the one on the right is a unidirectional gauge.

Pressure Gauge - Accessories
Safety Glass Front Safety Glass is normally used to prevent the glass shattering in the event of the bourdon tube rupturing. Liquid Filled Gauge The liquid filling is used to dampen any vibration/pulsation in the bourdon, either silicone oil or glycerin is used. Snubber Used 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. This slide describes the accessories very commonly used for pressure gauges. 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.

Pressure Gauge - Accessories
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 Tubes Used to dissipate heat by trapping condensed liquid to keep high temperature steam or condensing vapor from damaging the pressure gauge. These are some more pressure gauge accessories. PIG TAIL COIL PIPE

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. Diaphragm Seals are used with Pressure Gauges and Transmitters where isolation from the process fluid becomes necessary as in the case of corrosive fluids, for example. Please read the slide for more details on diaphragm seals.

Diaphragm Seals This shows the components of a diaphragm seal and a typical installation.

Pressure Gauge Selection Guideline
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 These are some guidelines to select an appropriate gauge for the required application.

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. Pressure gauges come in standard ranges so care should be taken to ensure that the right range is selected for the application. Here are some guidelines on choosing the best operating range of a pressure gauge.

Pressure Gauge Installation
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. Gas Service Liquid Service Here are typical installation details for pressure gauges. The top connection is preferred for gas installations and the side connection is preferred for liquid installations. Pressure gauges can also be installed on vertical pipe. The pressure gauge can be connected to the pipe by individual block and bleed valves or a two way manifold.

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 A Pressure Switch is mainly used for alarming but may also be used for interlocking purposes in a Safety Shutdown system. It can also be used for high or high-high or low or low-low actuation of pressure in the process . The set range can be adjusted within the switch range. The most commonly used sensing elements are either a Diaphragm or a piston.

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. Pressure switches can be used to sense either positive pressure or vacuum. They can either energize or de-energize a circuit depending on how they are configured. They can also be provided with a single or dual switches. Some dual switch pressure switches can be set up so that each switch can operate at a different pressure set point.

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. The switch housing can be specified to meet different conditions depending on whether they are mounted inside or outside or whether they are in a safe area or electrically classified area. They can also be provided with hermetically sealed switches or gold plated contacts. Pressure Switches are not commonly used today, since they are more likely to fail than transmitters and the failures can go undetected.

Pressure Transmitter The picture on the left is that of a typical transmitter with an integral indicator and the one on right shows the internal parts.

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. Please read through this slide to learn about the basic uses of a Pressure Transmitter.

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. This describes how a gauge pressure transmitter works.

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 Electronic pressure transmitters like the one on the left can provide three different types of outputs; a traditional 4 to 20 milliamp analog signal, a HART signal, which is a digital signal superimposed on the analog signal, and a fieldbus digital signal. Pneumatic transmitters, like the one on the right, provide a 3 to 15 P, S, I pressure signal.

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. Please take a few minutes to review the uses of a diaphragm seal system.

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. Basic uses of a diaphragm seal system.

Pressure Transmitter Selection Guideline
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 This gives the basic guidelines on selecting a Pressure Transmitter.

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. This is an example of a pressure transmitter installation detail.

Diaphragm Seal installation
This is an example of an installation detail of a pressure transmitter using a diaphragm seal system.

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.

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.

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.

Any Questions??? QUESTIONS
This completes the Control Systems Training Module CSE156.1 Flow Instruments. Are there any questions?