1. G.H.PATEL COLLEGE OF ENGINEERING AND TECHNOLOGY Instrumentation and Process control Code – 2150504 Topic:-Pressure Measurement : Liquid column elements, Elastic element gauges

2. Presented By :- Nirmal Patel- [130110105019] Parishwa Parikh-[130110105020] Archit Patel- [130110105021] Chintan Patel- [130110105022] Dhruv R. Patel- [130110105023] Dhruv V. Patel- [130110105024] Guided by:- Prof. Dharmesh ka. Patel

3. Index Barometer U-tube manometer Well manometer Enlarged leg manometer Inclined leg manometer Burdon pressure gauge Diaphragm pressure gauge Capsule pressure gauge Bellows pressure gauge Bibliography

4. 1.BAROMETER Barometer is used to measure atmospheric pressure. Atmospheric pressure is the pressure exerted by the air surrounding the earth, that goes on decreasing away from the earth surface. Principle: Barometer liquid balances the atmospheric pressure against vacuum and pressure hence reading is obtained in absolute units.

5. Construction and Working: Barometer has a glass tube closed at one end and open at the other. The length of the tube must be greater than 76.2cm. The tube is first completely filled by mercury and open end is temporarily plugged. Then tube is inverted so plugged end is immersed in a mercury pan amount, creating a vacuum at the top of the tube above the mercury column.

6. The mercury stabilizes at a certain level inside the tube and then reading ‘h’ is noted. The reading ‘h’ is proportional to atmospheric pressure acting on mercury in the pan. Note that this atmospheric pressure reading is in absolute units. We have stated that vacuum is present on the top of the tube above mercury, but actually there is vapor pressure of mercury acting on the mercury. Pressure ‘P’ is given by P = 6.66 x 10 -3 h, where h is in cm and P is in kg/cm 2.

7. MANOMETERS Principle: All manometers work on the effect of hydrostatic pressure exerted by a liquid column in manometer unknown pressure is determined by balancing it against some known pressure or vacuum.

8. 2.U-TUBE MANOMETER The U-tube manometer consists of glass U-tube partially filled with a suitable liquid like water, mercury etc. One of the arms or legs of the manometer is connected to unknown pressure tap to be measured, while others connected to other pressure tap or it is left open to atmosphere. When there is difference of pressure between two arms of the manometer, liquid levels in the two arms do not match.

11. Measurement of differential pressure: the two pressure taps are connected to two arms of the manometer so that height difference ‘h’ is proportional to difference between two pressures. Measurement of gauge pressure: when the unknown pressure is fed to one of the arms and the other arm is exposed to atmosphere, then ‘h’ represents the unknown pressure is in gauge units. Measurement of absolute pressure: when the unknown pressure is fed to one of the arms and the other arm is evacuated, then ‘h’ represents unknown pressure in absolute units.

12. 3.WELL MANOMETER

13. This is nothing but a U-tune manometer, modified by replacing one leg by a large diameter well. Usually well area is 300 times greater than the tube area. Due to this any change in pressure P 1 has negligible effect on manometric liquid level in the well is assumed to be fixed and it is called zero level. When pressures P 1 and P 2 are applied as shown in figure, the height “h” of manometric liquid in the tube is measured from zero line.

14. This height “h” represents differential pressure (P 1 - P 2 ).The static balance equation can be written as: P 1 – P 2 =ρh(1+(A 2 /A 1 )) Where, A 1 = area of the well A 2 = area of the tube

15. LIMITATIONS The non-uniformity of the tube may produce error in measurement. For getting high accuracy, zero level of the well is set at the zero level of the scale.

16. 4.ENLARGED LEG MANOMETER

17. In this manometer both the legs are enlarged in production as shown in figure and they are connected by a separable tubing. float is placed in one of the chambers. When pressures P 1 and P 2 are applied as shown in figure, the float gets displaced. This float displacement measured from zero line represents the differential pressure (P 2 –P 1 ). The manometer can be made recording type by connecting pointer mechanism to the float, that makes the pointer to deflect on the calibrated scale. The static balance equation can be written as: P 1 – P 2 = ρd(1+(A 1 /A 2 )), where A 2 > A 1

18. ADVANTAGES Recording type manometer. The span can be changed by using different size tubes. For recording type manometer mercury level need not be viewed, hence metal arms can be used that permit its use for measurement of high pressures up to 5000 psi.

19. 5.Inclined leg manometer The construction is very similar to enlarged leg manometer except that small diameter tube is inclined to the vertical axis. When pressure P 1 and P 2 are applied as shown in figure, then liquid rises in the tube. The level of manometric liquid inside the tube is measured from zero level along with the inclined tube; which represents the differential pressure (P 1 -P 2 ). The static balance equation can be written as: P 2 -P 1 = ρ*d*sinα(1+A 1 /A 2 )

21. Advantages Due to inclined leg, the manometer readings gets amplified. Hence it can be used for measurement of low pressure which cannot be measured by other manometers. By reducing angle α, the scale length and hence the sensitivity can be increased.

22. Manometric Liquids Desirable properties of good manometric liquid are low freezing point, high boiling point, non-wetting characteristic, low surface tension, chemically inert, clear visible interface and ability to maintain density at various temperatures. Manometric fluids used in practice are: 1.Mercury : It has low F.P (-38°F) and high B.P (675°F), but it corrodes metals and it is poisonous and expansive. 2.Water and coloring agents : to reduce surface tension coloring agents are used, which reduces capillarity effect in manometer. 3.Benzene, kerosene, CCL 4, toluene etc.

23. Calibration Manometer is subjected to known differential pressure and corresponding height difference is noted. The calibration curve can be prepared by plotting height difference versus differential pressure. This curve can be used to get the differential pressure for certain height difference.

24. Sources of error Temperature effect : Rise in temperature causes decrease in manometeric liquid density that affects the calibration which leads to an error. Capillary rise : To avoid rise effect, the tube diameter should be over 10 mm, otherwise capillary rise results in error in pressure reading. Meniscus shape : For water, the free surface is concave, while for mercury surface is convex. The level of manometric liquid should be noted at the centre of the miniscus.

25. Advantages Simple, inexpensive construction. High accuracy and sensitivity. Can be used for low pressure measurement. Desired span can be obtained just by using suitable manometric fluids. Pressure range of manometer is 3 to 100 KPa

26. limitations No over range protection. Requires large space. Non-portable. Levelling is required. Condensation of test liquid affects the readings.

27. 6.Burdon pressure gauge E. Bourdon introduced Bourdon tube in 1852 as a curved twisted tube having circular transverse section. According to Bourdon theory, a tube having internal cross-section that is not a perfect circle if bent or disorted, has the property of changing its shape with internal pressure variation. This causes the free and deflection of the tube which can be taken as the measure of change in pressure inside it.

28. Construction It uses different types of Bourdon springs as shown in figure. C-shaped Bourdon tube is formed by winding the tube to form segment of a circle having arc-length of 270°. In spiral type, number of turns are wound in the shape of a spiral about a common axis as shown in figure. In helix type, number of turns are wound in helix form. In figures ‘P’ indicates direction of application of pressure and ‘T’ indicates tip travel for rise in pressure.

31. the gauge consist of a C-shaped Bourdon tube, tip, adjustable link, segment lever, sector, pinion, spring and pointer. A C-shaped Bourdon tube is a thin-walled tube having a non- circular or nearly elliptical transverse section as shown in figure. One end of tube soldered or welded to a socket at the base through which pressure is fed inside the tube, while other side is sealed by a tip.

32. Materials A Bourdon spring can be made of any metal or alloy that exhibits satisfactory elastic properties. Materials used are brass, phosphor bronze, monel, beryllium, copper, stainless steel, etc.

33. Pressure ranges C-shaped tube : 0 to 1,00,000 psi. Gauge pressure : 0 to 12,000 psig (0.83 Mpa) Absolute pressure : 0 to 100 Pisa (0.07 Mpa) Vacuum : 0 to 30” Hg.(vac.)

34. Working When fluid under pressure to be measured enters the Bourdon tube, its cross-section tries to become more and more circular that causes the free end to deflect, that is called as tip travel. The amount of tip travel for given rise in pressure is a function of tube length, wall thickness, cross-section geometry and elastic modulus of the tube material. This linear tip travel is guided and amplified by adjustable link and segment lever and then it is given to sector and pinion arrangement. ector and pinion convert the amplified tip travel into proportional rotary motion of the pointer connected to the pinion. The pointer deflection can be read on the scale calibrated in terms of pressure.

35. Advantages Low cost and simple construction. Wide pressure range. High accuracy in relation to low cost.

36. limitation Low spring gradient. Susceptibility to shock and vibration. Bourdon tube material possesses some hysteresis in a pressure cycle. Hysteresis can be kept minimum by proper heat treatment and by using proper materials.

37. 7.Diaphragm pressure Gauge 1)Principle : when pressures are applied on either sides of the tight diaphragm, then it gets deflected. This deflection of the diaphragm is proportional to the differential pressure across it. Thus differential pressure across the diaphragm can be measured in terms of its deflection. 2) Construction and Working : (i)Metallic diaphragms : these elements are flexible circular discs, either flat or corrugated. These elements convert pressure signal to pointer deflection.

39. The metal is heat-treated before forming a diaphragm to produce maximum elastic limit. After forming, the diaphragms are heat-treated to relieve internal stresses. A diaphragm is usually designed so that the possible over a spec Deflection – versus- pressure characteristics are as linear as possible over a specified pressure range and with a minimum of hysteresis and minimum shift in the zero point. Linearity and sensitivity are determined to a great extent by by the depth and number of corrugations and the angle of formation of the diaphragm face. The sensitivity can be increased by increasing the number of corrugation and by decreasing the depth of corrugations and with a sacrifice in linearity. The maximum sensitivity can be obtained by using flat, non-corrugated diaphragms. The diaphragm deflection with pressure is dependent on factors like diameter, metal thickness, shape of corrugations, number of corrugations, modulus of elasticity and applied pressure. The deflection varies with fourth power of diameter, hence if diameter is doubled, the deflection increases 16 times for given pressure change.

40. The diaphragm material should be chosen such that maximum deflection at the centre is one third of its thickness so as to keep material within elastic limit. Diaphragm seals are used to prevent the contact of process fluid with diaphragm which is necessary to : (i)protect the corrosion and clogging of the diaphragm. (ii)prevent the loss of explosive or hazard process fluid in case of failure or replacement of the diaphragm. Metals used are brass, phosphor-bronze, beryllium-copper, stainless steel.

41. (ii) Non- metallic or Slack diaphragms: these diaphragms are very flexible and hence they are used in low- pressure and vaccum gauges. Materials used : Slack diaphragms are made from variety of materials like synthetic rubber, neoprene, leather, elastomers reinforced by cotton, nylon, decron. The choice of material depends upon temperature and composition of the process fluid in contact with the diaphragm. The diaphragm deflection is opposed by a light spring.

43. (3) Calibration: Calibration procedure is same as that for Bourdon gauge. (4)Advantages and Limitations: Advantages: 1. Moderate cost and small size. 2. Corrosion resistant materials can be used. 3. Good linearity. 4. can be used for pressurized slurries. Limitations: 1. Lack of vibration resistance. 2. Suitable for relatively low pressure. 3. Troublesome repairing.

44. 8.Capsule Pressure Gauge (1)Principle: The principle of capsule is same as that of the diaphragm element. (2) Construction and Working:2 A capsule is formed by welding or soldering of two diaphragms at their periphery. Number of such capsules are arranges in the stack form as shown in fig. Pointer mechanism is connected to connected to topmost capsule that represents pressure on the calibrated scale. When pressure inside the first capsule changes, the free end of the last capsule in the stack gets deflected. This deflection depend upon ( I ) diameter of the capsule (ii)thickness of the material (iii)elasticity of the diaphragm material (iv) shape and number of corrugations on the diaphragms. The deflection versus pressure pressure relationship is almost linear and any non- linearity can be compensated.

46. Measurement of gauge pressure or vaccum : Unknown pressure is fed inside the capsule, while its outside is exposed to atmosphere so that reading would be in gauge or vaccum units as shown in fig. Measurement of absolute pressure : Unknown pressure is fed outside the evacuated capsule so that reading would be in absolute units. Materials used : Phosphor bronze, stainless steel, Ni-SPAN C Pressure range : The range depends upon number of capsules in the stack. Phosphor bronze capsule : 0 to 0.5” water to 0 to 30 psi. stainless steel : 0 to 8” water to 50 psi. Ni-SPAN C : 0 to 4” water : to : 0 to 30 psi. Maximum pressure range is 0 to 1000 psig and Maximum vacuum range is 0 to 30” Hg (3) Calibration: calibration procedure is same as that for Bourdon gauge.

47. (4) Advantages, Limitation : Advantages : In addition to all advantages of diaphragms pressure gauge, capsule gauge provides larger deflection for relatively small pressure changes. Limitation: The gauge is suitable for relatively low pressures. The capsule has hysteresis effect of 0.25 to 0.5% of full scale. Hysteresis effect can be defined as the difference between the up-scale deflection and down-scale deflection, measured at the same applied pressure.890

48. 9.Bellows Pressure Gauge 1)Principal: Bellows element is the elastic element which gets deflected at the free end when pressure is fed through fixed end. This free and deflection can be taken as the measure of pressure inside it.

50. Construction and Working A bellows element is a one-piece collapsible, seamless metallic unit having several convolution or folds and it is formed from a very thin-walled tubing. Bellow element is manufactured either by turning from a solid stock or by welding stampled annular rings. Bellows may also be manufactured by welding a series of formed plates together at their inner and outer diameters. The bellow element resembles the bulb of household kerosene pump.

51. 2)Materials used : bellows are made of brass, phosphor bronze, beryllium, copper, monel, stainless steel. the choice of material depends upon pressure range and corrosion resistance.

52. 3)Working : when pressure is fed inside the gauge,it acts on the outside of the bellows and bellows get compressed. Due to compression, free end of the bellows moves against the opposing force of the spring. This free end movement raises the rod against it, that transmits the movement to the pointer. This pointer deflection represents the pressure fed outside the bellows. The bellows movement for given pressure can be increased by increasing bellows diameter, while stroke length should be within 10% of the maximum stroke so as not to exceed the elastic limit of the material. Bellows motion is restricted by the opposing spring. Maximum pressure rating for the bellows is the value of pressure that developes maximum allowable stress in given bellows. Maximum stroke rating for the bellows is the magnitude of stroke which developes the maximum allowable stress.

54. 4)Measurement of gauge pressure : The arrangement shown in fig with inside of the bellows exposed to atmosphere, can be used for measurement of pressure in gauge units.

55. 5)Measurement of absolute pressure: for absolute pressure measurement, two-bellows system is used as shown in fig. One of the bellows is completely evacuated and acts in opposition to the measurement bellows. The pointer deflection represents absolute pressure inside the measurement bellows.

56. 6)Bellows different pressure gauge : one of the pressure is fed inside the bellows, while the other it, so that the differential pressure acts across the bellows. This deflection is communicated to the pointer through sealed shaft as shown in fig.

57. 7)Pressure Ranges : Gauge pressure- 0 to 2000 psig vacuum - 0 to 30”psig Differential pressure- 0 to 50 psi at static pressures up to 2000 psig

58. 8)Calibration : calibration procedure is same as that for bourdon gauge.

59. 9)Advantages, Limitations : Advantages 1.Moderate cost 2.Adaptability for absolute and differential pressures. 3.Low to moderate pressure range.

60. Limitations 1.Not suitable for high pressures. 2.Requires ambient temperature compensation.

61. Bibliography Instrumentation and process control by A.P Kulkarni (nirali prakashan) Wikipedia Google images