Differential Pressure Transmitter (Pneumatic Kent-Tleghl)

The pneumatic pressure transmitter is an element of the regulation circuit pneumatic that transduces the greatness to be measured. In this case, a pressure in a modulated air pressure of 3 to 15 psi. Figure 1 below shows the details.

Figure 1 - Pneumatic Pressure Transmitter

Components:

1. Measurement cell
2. Inelastic membrane
3. Leverage of forces
4. The fulcrum of the lever (3)
5. Lamina of the modulator group
6. The nozzle of the modulator group
7. Throttling of the modulator group
8. Amplifier relay
9. Reaction bellows
10. Zero adjustment spring
11. Auxiliary lever of the reaction bellows
12. Transmission field adjustment slider

This transmitter is of the balanced-force type. The transmitter is supplied with a constant compressed air pressure of 20 psi and outputs a modulated pressure (Pu) ranging from 3 to 15 psi (depending on the differential pressure to be transmitted). The differential pressure to be transmitted (P1 - P2) is sent to a measuring cell (1).

An inelastic membrane divides this cell into two chambers (2). The highest pressure (P1) operates in a chamber (+), and in the other chamber (-), the lower pressure (P2). A force (Fm) directed upwards acts on the membrane. The greater the pressure, the greater the differential pressure (P1 - P2). The membrane is connected to the left end of the force lever (3), whose fulcrum is at point (4).

Force on the lever (3) will act as an upward force directly proportional to the differential pressure. The force F generates a moment M given by:

F_m = F_m X a

Where a is his arm, which tends to bring the lamina (5) closer to the nozzle (6) and then increase the modulated pressure from the modulator group. The pressure modulated and amplified by the relay (8) is sent to the output, and the reaction bellows (9).

The outlet pressure (P_u) generates in the reaction envelope a force (F_u) which in turn creates a moment (F_m) on the right end of the force lever (3) given by:

F_m = F_u X b

where b is its arm, which tends to move the plate away from the nozzle and decrease the outlet pressure.

The system is in equilibrium when the two moments are equal. Any unbalance involves bringing the foil closer to the nozzle and then changing the outlet pressure (P_u).

In conditions of equilibrium, we have:

F_m X a = F_u x b

Since the Fm is proportional to the differential pressure (P1 - P2) to be transmitted and the Fu is proportional to the outlet pressure (Pu), it has that at each value of the differential pressure corresponds to a well-defined value of the pressure output of the transmitter.

By varying the arm (b) of the reaction force (Fu) and moving the slider (12), the transmission range is changed. The spring (10) (zero adjustment spring) acting on the auxiliary lever (11) allows obtaining an outlet pressure of 3 psi when the differential pressure is zero.

Construction details 

The instrument consists of the following fundamental parts:

• Measurement cell
• Basement
• Membrane
• Leverage of forces
• Modulator group
• Amplifier relay
• Reaction group
• Zero adjustment spring
• Zero suppression spring

Figure 2 - Construction details

The individual parts are:

• 1. Cella di misura
• 2. Membrana
• 3. Effetto leva delle forze
• 4. Messa a fuoco della leva di forza 131
• 5. Gruppo modulatore lamella
• 6. Ugello gruppo modulator
• 7. Limitazione
• 8. Relè dell'amplificatore
• 9. Soffietti di reazione
• 10. Molla di regolazione zero
• 11. Leva ausiliaria
• 12. Cursore di regolazione del campo di trasmissione
• 13. Prigionieri della cella di misura
• 14. Corpo superiore della cella di misura
• 15. Corpo inferiore della cella di misura
• 16. Valvola di sfiato
• 17. Valvola di sfiato
• 18. Dado di bloccaggio della piastra 1221 alla leva di forza 131
• 19. Flange di collegamento
• 20. Supporto del trasmettitore
• 21. Staffa di ancoraggio
• 22. Piastra di fissaggio membrana (2) alla leva di forza (3)
• 28. Controdado del cursore di regolazione (12) della gamma di trasmissione
• 29. Vite micrometrica per la regolazione della posizione della piastra (5)
• 30. Slitta portaugello (6)
• 31. Vite slitta portaugello (30)
• 32. Vite fissaggio portaugello 1301
• 33. Fulcro della leva ausiliaria (11)
• 34. Vite regolazione molla regolazione zero
• 35. Coperchio
• 36. Molla di soppressione zero
• 37. Vite di controllo della molla di soppressione dello zero (36)

Consider figure 3, which represents the entire sectioned transmitter. The measuring cell is delimited by the upper body (14) and the lower body (15) of the transmitter, joined together by the studs (13). The membrane (2) is clamped between these two bodies. The upper body and its membrane delimit the chamber - the lower body and the membrane delimit the chamber +.

The chambers are equipped with a vent valve (16) with the relative needle cap (17) (the vent valve of the chamber is not visible in the figure positive). When the process fluid is a gas, the vent valve is mounted laterally in order to allow the condensation to drain. The differential pressure connections to be transmitted to the measuring cell are made using the connecting flanges (19). The materials used to construct the measuring cell vary according to the nature of the process fluid.

Basement

The lower body (15) of the measuring cell is rigidly fixed to the base (20) of the transmitter. At the bottom of the basement below the measuring cell is fixed the anchor bracket (21). The instrument under test is designed to be mounted on a pipe in a horizontal position.

However, it is possible to mount the transmitter on a pipe in place vertically; in this case, it is necessary to calibrate it in a vertical position. Figure 4 shows the cross-section of the membrane (2)

Figure 4 - Membrane of Pressure Transmitter

• B. Diaphragms
• C. Pin
• D. Silicone oil
• E. Ring gaskets
• 2. Membrane
• 22. Diaphragm fixing plate (3) to the force lever (3)

It consists of two diaphragms (B), appropriately shaped and very thin of steel, whose outer circumference is welded to solid wood (A). In the central part of the solid wood (A), a hole is made, in which it can move a pin (C) vertically. The membrane section shows how the inner circumference of the diaphragm is fixed to the pin (C).

Silicone oil (D) is interposed between the two diaphragms and the solid wood. The oil can pass freely between the two chambers delimited by the two diaphragms. The two-ring gaskets (E) avoid irremediable permanent deformations of the two diaphragms. Where there is a large difference in pressure on the external surfaces of the two diaphragms, the pin lowering or rising meets one of the two seals and blocks communication between the two chambers. The differential pressure acting on the outer surfaces of the two diaphragms causes the movement of the pin, which in turn transmits it to the lever of the forces (3), being integral with the latter using the plate (22). The plate is fixed to the lever (3) through the nut (18) (observe the entire transmitter). The diameter of the diaphragm and the thickness of the diaphragm depends on the field transmitter transmission.

The diameter of the membrane will be greater the smaller the differential pressure to be transmitted. The membrane is made of stainless steel for high and medium differential pressures. When the differential pressure is low, it is made of polyethene (10 ± 150 mmH2O) The transmission range of the transmitter in question may vary from 0 ~ 500 to 0 - 6000 mm H2O. The maximum static pressure that can bear is 100 Kg/cm².

Figure 5 - Leverage of forces

• 3. Leverage of forces
• 4. Fulcrum of the lever of forces
• 12. Transmission field adjustment slider
• 15. Lower body of the measuring cell
• 23. Circular flexible diaphragm
• 24. Lever fixing flange (3)
• 25. Ring 26. Thrust bridle
• 27. Attacks of attack of the thrust bridle
• 28. Cursor lock nut (12)
• 56. Ring gaskets

Figure 5 on the side represents the detail of the lever of the forces (3) and its fulcrum (4). The force lever, coming out of the lower body (15), consists of two parts screwed together at the exit point from the measuring cell, consisting of the fulcrum point (4). The lever's fulcrum is made of a flexible circular diaphragm (23) of steel.

The diaphragm is tightened, in the central area, between the two parts of the lever and peripherally between the flange (24) and the ring (25). Between the diaphragm and the ring (25) and between the ring and the lower body (15), there are interposed Teflon ring gaskets (56). Therefore, the diaphragm also provides for sealing the process fluid in the measuring cell. The lever has two thrust harnesses (26) made of two flexible steel plates parallel to the base of the transmitter. The thrust bridles have one end integral to the base and the other connected to the lever's two appendices (27) (3). The external part of the force lever is threaded; in this way, it is possible to move the slider (12) of the transmission field adjustment after loosening the lock nut (28).

Modulator group

Figure 6 - Modulator group

The modulator group is of the simple type. The plate (5) is integral to the nozzle holder slide (30). Small manual corrections of its position can be made using the micrometre screw (29). The screw (29) also has the task of transmitting the displacements of the force lever to the blade (5). The nozzle (6) is fixed to the nozzle holder slide (30). The nozzle holder slide can be translated vertically relative to the base of the transmitter using the screw (31) after loosening the locking screws (32). The restriction (7) is screwed onto the base part that serves as a collecting plate.

The relay (8) is a continuous leakage air pressure and volume amplifier (see test 4).

The reaction bellows (9) has the fixed base integral with the base of the transmitter and the mobile one mechanically connected with the auxiliary lever (11). The auxiliary lever is pivoted through the cross fulcrum (33). The lengthening of the reaction bellows; therefore, the raising of the auxiliary lever is transmitted to the force lever (3) using the slider (12).

The zero adjustment spring (10), which allows obtaining at the outlet from the transmitter a pressure of 3 Psi, when the pressure to be transmitted is null, acts on the auxiliary lever (11) and is adjustable using the screw (34). To perform the zero adjustments without removing the cover, a hole is made in the lower part of the cover (35) and correspondence of this screw.

The visual examination of this transmitter can be extended to all parts that compose it. The examination makes it possible to ascertain the efficiency of each component. After visual inspection, the pre-calibration and calibration of the transmitter must be carried out.

 Visual inspection of the measuring cell with its membrane

Figure 7 - Measuring Cell with its membrane

 Visual inspection of the Force Lever

Figure 8 - Force Lever

 Leverage of forces

Visual inspection of the measuring cell

Figure 10 - Diaphragm

Components:

• 2. Membrane
• 3. Leverage of forces
• 13. Prisoners
• 14. Upper body of the cell
• 15. Lower body of the cell
• 18. Nut that locks the fastening plate (22) of the diaphragm (2) to the lever of forces (3)
• 20. Basement
• 22. Diaphragm fixing plate (2) to the force lever (3)
• 38. Tightening nuts of the upper body of the measuring cell
• 39. Membrane teflon seal (2)
• 40. Locking washers

The examination of the measuring cell is reduced to checking the efficiency status of the membrane (2); that is, there are no oil leaks due to diaphragm breakage.

It should be noted that permanent membrane deformations are found only by testing the linearity of the transmitter outlet pressure.

Sometimes it is necessary to remove the measuring cell to clean it from any more stations due to the sedimentation of the process fluid.

Procedure to remove the membrane

• Insert a 1/4 "Allen key into the pressure inlet port from transmit in the lower body of the measuring cell (15), and loosen the lock nut (18).
• Remove the nuts (38).
• Remove the upper body (14) of the measuring cell from its studs (13).
• Mark the diaphragm and the lower body (15) with a notch measuring cell so it can be put back in the same position during the assembly phase.
• Remove the membrane from the lower body housing.
• Generally, the membrane and the lower body are marked by the manufacturer with a stamp.

Recommendations to reassemble the membrane

  Figure 11 - Diaphragm

The two Teflon seals (39) must be intact.

If the diaphragm is new, it must be marked on its housing position in the lower body of the measuring cell.

Make sure that the two lock washers (40), the plate (22) and the force lever (3) are in the position shown in figure 11 on the side.

The lock nut (18) must be tightened after making sure that the lever (3) of the forces is perfectly horizontal.

 

Components

• 3. Leverage of forces
• 18. Plate lock nut (22) at the lever (3)
• 22. Plate for fixing the diaphragm to the lever (3)
• 40. Locking washers

 

 Examination

Figure 6 - Modulator group

 

Components:

• Modulator group
• ReIé amplifier
• Reaction group
• Zero adjustment spring
• Zero suppression spring

Modulator group

The nozzle (6), the lamina (5) and the restriction (7) must be kept perfectly clean.

In order to clean the nozzle and the lamina, proceed in the following way:   Loosen the screw (50) of the plate (49) connection and (48) of the nozzle. Move the plate itself to free the connection Unscrew the nozzle (6).   Remove the nozzle supply tube (51) from the nozzle holder slide (30), pulling it down. Thoroughly clean the foil with cotton dampened with alcohol or another suitable solvent. Taking care not to bend it. Clean the nozzle by inserting a wire of copper or brass, with a diameter maximum of 1 mm. After immersing it in a solvent, blow it with clean compressed air in order to dry it. Reassemble it on its slide (30).   Insert the nozzle connection (43) into its housing, being careful not to damage the seal. Reposition the plate (49) and secure it with the special screw (50).

The bottleneck must be kept clean.

The bottleneck can be examined and cleaned as follows:   Unscrew the restriction (7) and clean it by inserting a wire with a maximum diameter of 0.15 mm. Lubricate the ring gaskets with vaseline, and reassemble the restriction.

See Figure 12 below

Figure 12 - Assembly

Failures and remedies

Figure 2 - Construction details

Special applications

Figure 13 - Special applications

Zero suppression

Differential to be transmitted, so the so-called suppression is necessary of zero. This suppression consists in applying a spring (36), used in compression, at the free end of the force lever (3). In this way, the lever acts as a fixed force due to the suppression spring. Therefore, the force due to the differential pressure will succeed in moving the lever only after overcoming the suppression spring's resistance. The outlet pressure will assume a value of 3 psi when the pressure differential has assumed a value equal to the minimum fixed, using the deletion.

Figure 14 - Special applications

Inversion action

In case you want to have the maximum outlet pressure (15 psi) when the differential pressure is zero and the inversion of the action is carried out. This version can be made by sending the P1 in the chamber and P2 in the + chamber and applying it to the free end of the force lever (3), a spring (36) used in traction. This spring should be calibrated when the differential pressure output is cancelled, assuming the value of 15 psi.