GB2181552A - Pressure transmitter - Google Patents

Abstract

A pressure transmitter comprises a chamber (7a) (7b) partially bounded by a moveable valve member (15). The chamber has an inlet (17) and an outlet (18) for fluid under pressure. The valve member is in the form of a wall (15) adapted to move by a bellows action relative to the outlet (18). When the external pressure on the valve member e.g., from liquid in a tank, is greater than that in the chamber the wall is urged against the outlet allowing a build up of pressure in the chamber to the value of the external pressure. At this point the wall is pushed by the increased pressure out of contact with the outlet which allows fluid to bleed through the outlet to maintain a substantially equilibrium pressure in the chamber. This pressure is measured via a further outlet from the chamber. In a preferred embodiment a temperature sensor is incorporated in the device. <IMAGE>

Description

SPECIFICATION Pressure transmitter The present invention relates to pressure transmitters which may be used as pressure gauges or liquid level indicators.

According to the present invention there is provided a pressure transmitter comprising a chamber partially bounded by a moveable valve member, the chamber having an inlet and an outletforfluid under pressure, wherein the moveable valve member is a wall adapted to move by a bellows action relative to the outlet between a position in which the chamber outlet is substa ntia lly blocked by contact with the wall and a position in which the outlet is open,the arrangement being such that, when the pressure on the valve member outside the chamber is greater than that in the chamber, thewall is in position againstthe outlet which allows a build up of pressure in the chamberto a value equal to that outsidethe valve member, whereupon the valve member loses contactwith the outlet allowing fluid to bleed through the outletto maintain a substantially equilibrium pressure in thechamber,the chamber having a second outletthrough which the pressure inside the chamber can be read.

Preferably the transmitter comprises a shank in which a spindle is mounted, the spindle being sealed to the shank at one end thereof and the other en being spaced from the shank by a space which space is in flow communication with the area contained by the valve member and defines part of the sald chamber. The outlet may beformed in the nose ofthe spindie and the fluid leaves the chamber passage through the spindle.

Preferablythespindle is moveable relativetothe valve member in a direction longitudinally of the transmitter and isfixable in a selected position.Adju- stment of the relative axial positions of the spindle and shank may provide a useful method of varying the sensitivity of the transmitter.

Advantageously the transmitter additionally comprises tem peratu re sensing means disposed to read the temperature of a portion of thetransmitterthe temperature of which will be substantially equal to that of the mediu m the pressu re of wh ich is being determined.

Such a transmitter could usefully replace existing "sealed pressure transmitters" which are currently used to determinethe pressure of the fluid contents of a tank. The valve memberwould be located in the tank and the of the shank would by outside the tank anabling accessto be madetothevarious pressure ports.

The invention will now be described, byway of ex ample, with referance to the accompanying drewing which is a longitudinal cross-section of a pressure transmitter according to the present invention.

Referring to the drawing, there is shown a pressure transmitterwhich is slightly mushroom shaped hav ing a head 2 and a stem 3. The stem, which is cylindri cal, comprises a shank4which is an open ended tube having atthe head end of the stem 3 a wider internal diameter portion 4a and attheotherend a reduced diameter portion 4b. A generally cylindrical spindle 5 having an outer diameter equal to the reduced internal diameter 4b of the shank thus enabling a seal to be made between the spindle 5 and the reduced diameter portion of the shank4. Agrub screw6 is located in a screwthreaded hole 6' in the shank4and may be tightened on to the spindle when the required position of the spindle 5 in the shank4 has been achieved. The annular space formed between the spindle 5 and the wider diameter portion 4a of the shankforms part of a chamber7a.

A nose 8 is either integrallyformed with the spindle, or, as shown, fitted into the head end ofthe spindle 5. In the present embodimentthe nose 8 is a hexagonal-headed bolt, the maximum diameter of which is equal to the wider internal diameter of the shank4. Theflatsurfaces on the boit headform channels 9with the shank4which channels connectthe part of the chamber 7a with a second part 7b in the head of the transmitter 2.

Accessto the chamber7a is given through two screwthreaded apertures (one shown at 17) in the shank. Although the apertures are shown as screw threaded any method of pipe connection can be used. The aperture 17 forms the inist for air under pressure to the chamber 7 and will be connected via airlines 10 to an aircompressorvia aflowregulator (not shown).The second sperture, which is not shown in the drawings is connected to a pressure reading device for reading the presaure at anytime existing in the chamber, particularly an equilibrium pressure It wil I be apprecaited that a ny suitable fl uid could replace the air in the transmitter.

The head 2 of the transmitter comprises a mount 12 to which the shank is fixed, the shank having a narrower diameterend 4c to which the mount 12 is secured by a method such as welding. The mount 12 is generally cylindrical having a larger diameter portion which bears against a tank wall 13 via a washer 14, and smaller diameter portion.A valve member 15,16 encloses a space 7b bounded also bythefront end of the mount 12,which space being inflowcom- munication with the space 7a as already described, the two spaces and the channels connecting them forming the chamber7 ofthetransmitter. The valve member is stainless steel and comprises a circular wal l 15 which is secu red to the valve mem ber throughout its circumference by a method such as brazing.

The steel is shaped as a concertina with two bends of the metal and this forms a resilient bellows portion 1 6, the final length of which infixed to the mount 12, for example by brazing th roughout the circumference at 1 6b.

The nose 8 of the spindle 5 is formed with a bleed nozzle 18 which acts as the outletto the chamber 7.

The bleed nozzle 18 leads directly into a wider diame tercylindrical channel which extends longitudinally through the spindle 5 and exhausts to the atmos phere at the rear end of the spindle. The spindle 5 extends outside the shank 4 and this can be extended to form a remote vent by use of a connector 19.

To assemblethetransmitterthe spindle is moved intotheshankuntil ittouchesthewall exerting no force on either component. The spindle is then locked in position by tighteni ng the g ru b screw 6 againstthespindle 5.The relaxed position of the wall 15 substantially blocks the bleed nozzle 18.Therewill always be a slight seepage from the chamber 7via thevleed nozzie.Thewall can moveawayfromthe bleed nozzle in the event of an increase in pressure in the chamber 7, the natural resilience in the bellows portion 16 of the member enabling the wall 15 to move backto the nose automatically as a result of a pressure drop occuring across the wall between the tank and the chamber.

Thetransmitteris mounted through an aperture in a tankwith the head inside the tank and most of the shank, including particularly the chamber inlet and chamber pressure reading access outsidethetank. A rubber washer 14 is between the mount 12 and thetank wall 13 to seal the transmitter to the tank and a lock nut 22 tightens against the outside ofthetank, optionallythrough a second washer 23 by travel along a screwthreaded outer portion 24 of the shank.

The operation of the transmitter is as follows: air under pressure is supplied to the chamber7 through the inlet port 17. If the pressure in the tank is greater than that in the chamber 7 the wall 15 will substantially block offthe outlet 18 so thata build up of pressure occurs in the chamber 7. When there is an equilibrium between the pressure in the tank and that in the chamber, the wall 15 is pushed away from the nose 8, unblocking the outlet 18 so thatexcessfluid can leave the chamber, thus maintaining in the chamber a fairly constant pressure which is equal to that in the tank. This pressure can be measured via the second port 17(not ilustrated) by conventional means.

As an optional extra itis possible to combine in a single unit a pressure sensing device and a tem perature sensing device. This is achieved by incorporating a temperature sensor in the transmitter already described. By virtue of its permanent contact with the liquid inthetankwhenthedevice is in use, the mount will be at substantially the same temperature as the liquid. The effect of the flow of fluid in the chamber 7 will substantially effect this. Therefore by taking the tem peratu re of the mount for the temperature of the contents of the tank reasonable accu racy is assu red. to make this possible the mount 12 is bored at 21.The shank4 is milled at 25 including the screwthreaded section, to provide a keyway for a temperature probe 26, the thermocouple end of which is inserted into the bore 21. To prevent the temperature probe from falling out of the keyway during assembleyoruse,the probe is bound into the keyway,forexample by a length of wire wrapped round the shank.

The probe26 is connected via a plug and socket27 to a transmitter (not shown) to enable the temperature to be determined in the usual way. The plug and socket 27 are also attached to the shank housed in a milled portion 28 of the shank.

By incorporating the temperature sensing in the same unitasthe pressure sensing the number of data retrival holes that need to be in the tank is reduced byat leastons. If thetemperature probe is included in each pressure transmitter more information is retrieved than is presently the case.

Claims (7)

1. A pressure transmitter comprising a chamber partially bounded by a mouveable valve member, the chamber having an inlet and an outletforfluid under pressure, wherein the moveable valve member is a wal I adapted to move by a bellows action relative to the outlet between a position in which the chamber outlet is substantially blocked by contact with the wall and a position in which the outlet is open,the arrangement being such that when the pressure on the valve member outside the chamber is greater than that in the chamberthe wail is in position againstthe outletwhich allows a build up of pressure in the chamberto a value equal tothatoutsidethe valve member, whereupon the valve member loses contactwith the outlet allowing fluid to bleed through the outletto maintain a substantially equilibrium pressure in the chamber, the chamber having a second outlet through which the pressure inside the chamber can be determined.

2. A transmitter according to claim 1 comprising a shank in which a spindle is mounted, the spindle being sealed to the shank at one end thereof and at the other end being spaced from the shank, the space so defined being in flowcommunication with the area contained by the valve member and defining partofthesaidchamber.

3. A transmiter according to claim 2 wherein the outlet is formed in the nose of the spindle and the fluid leave the chamber by passage through the spindle.

4. A transmitter according to claim 2 or 3 wherein the spindle is moveable relativetothevalve member in a direction longitudinally of the spindle and is fixable in a selected position.

5. Atransmitter according to any of the preceding claims also housing a temperature sensor located in the transmitter so as to sense indirectly the temperature outside the valve member.

6. A transmitter according to claim 5 wherein a temperature sensor is disposed in a keyway in the body of the transmitter with its temperature sensing head disposed in a mountwhich forms part ofthe head of the transmitter and which mount is, in use, in contactwith the fluid the temperature of which is to be determined.

7. A pressure transmitter substantially as herein described with reference to the accom panying d rawing.