GB2197071A - Pressure and temperature measurement in tanks - Google Patents

Abstract

A temperature unit (16) for attachment to a hydrostatic pressure probe (11) consists of a temperature sensor (24) connected to associated electrical wiring. The unit includes support for the temperature sensor in heat exchange relationship with the contents of the tank, but not in the gas flow which forms part of the hydrostatic pressure measurement. The sensor is housed in a chamber which may be out of the gas flow but in flow communication with the tank contents via aperture (29). The sensor may be mounted in a chamber in a pipe length which can be inserted between lengths of the probe (11): the chamber need not be in direct contact with the tank contents. <IMAGE>

Description

SPECIFICATION Pressure and temperature measurement in tanks The present invention relates to the measurement of temperature and pressure of the contents of tanks, and is particularly concerned with a temperature unit for attachment to a hydrostatic pressure measurement probe.

At present two methods are available for measuring the temperature of the contents of a tank. The first is by drilling a hole in the tank wall and inserting a temperature well housing a temperature probe. The disadvantage of this method is that it requires a special hole to be made and sealed. The second method uses a resistance probe element connected by wiring to equipment which evaluates the electrical signal received from the element and produces a result in whatever form is required by the system. The type normally used has a steel sheathed resistance probe with an integral electrical circuit also sheathed in metal. The probe is connected to the equipment via terminals in a box outside the tank.

The length of sheathed wire is fixed and so it is necessary to obtain in advance the correct length for the tank. This probe is inserted into the contents of the tank down a specially fitted tube in the tank and a specially drilled hole is made. The terminal box sits above the tank.

It was desired to create a composite unit for measuring temperature and pressure in the situation where hydrostatic probes were used to determine the pressure so as to remove the need for a separate hole in the tank for the temperature device and thus mitigate the problems encountered in sealing these holes.

Two solutions were considered. Pushing a temperature sensor down the centre of a hydrostatic probe is unsuccessful in a standard situation because of the effect of the much cooler pressurised gas which passes through the hydrostatic probe and will lower the reading. If the tube has a diffuser this situation is aggrevated as the temperature probe is not in contact with the liquid in the tank. In a system of pressure measurement using coaxial tubes the outer tube is shorter by as much as a third. The possible solution of mounting the temperature probe up the outer tube is therefore unsatisfactory in that it does not measure the temperature in the bottom third of the tank. It is important to measure this temperature as if there is setting of contents this is where it will happen.

Under some circumstances the contents of the bottom of the tank remain hard owing to the local temerature conditions. It can therefore be required to know the temperature above the level of the bottom of the hydrostatic probe.

According to the present invention there is provided a temperature unit for attachment to a hydrostatic pressure probe and containing a temperature sensor connected to associated electrical wiring, the unit comprising means for supporting the temperature sensor,in use, in heat exchange relationship with the contents of the tank but out of the gas flow associated with the hydrostatic probe.

The unit may be designed for attachment to the end of the hydrostatic probe in which case the incorporation of a diffuser for the hydrostatic probe gas may optionally be provided. If the unit is to be incorporated higher up the hydrostatic probe, a conduit for the gas through the unit is essential.

The heat exchange relationship may be a direct one by exposing the sensor to the contents of the tank, preferably by providing a chamber in the unit with access for tank contents. Alternatively the heat exchange relationship may be an indirect one created for example by mounting the sensor in contact with a heat-conductive material which is itself in contact with the tank contents. The unit is preferably constructed in heat conductive material with a conduit therethrough for hydrostatic probe gas and a chamber for containing the temperature sensor.

In one arrangement the unit constitutes a plug for the bottom of the section to which it is fitted and a sealed connection would normally be made.

Advantageously the temperature sensor is held in the unit by clamping.

The present invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a longitudinal cross-section of a temperature unit according to the present invention, Figure 2 shows a tube connection arrangement for the top of the assembly outside the tank to cater for measurement of temperature, Figure 3 is a longitudinal cross section of a second embodiment of the invention, and Figure 4 is a longitudinal cross-section of a third embodiment of the invention.

The temperature and pressure probe assembly illustrated in combination in Figures 1 and 2, 2 and 3 or 2 and 4 comprises a hydrostatic tube 11 having a T-junction piece 12 fitted to the top. On one side the junction is connected to a valve 13 by means of Which gas is supplied and the pressure sensed for the pressure measurement. The hydrostatic tube 11 is continued vertically above the junction piece 12 by a tube 14 terminating in a temperature sensing wiring and transmitter box 15.

At the bottom of the tube and fitted into the tube by tack welding is a composite cylindrical unit 16 comprising a diffuser 17 and a shield 18. The diffuser 17 comprises four to six shaped slots 19 which form passageways for gas under pressure which has beep sent down the tube 11. Underneath the diffuser the unit comprises a solid body 21 having a small central bore 22. At the bottom of the body 21 there is a threaded counterbore leading into the bore 22 and coaxial therewith. The space surrounded by the shield 18 forms a chamber in flow communication with the liquid in the tank by means of apertures 29 which ensure that the temperature of the liquid in the chamber is substantially the same as that of the surrounding liquid.The shield 18 which is cylindrical and welded to the outside of the body 21 below the diffuser also forms some protection against accidental damage for the sensitive temperature sensor fitted therein.

The temperature sensor comprising a resistance probe 24 and associated electrical wiring 25 is fitted into the unit with the probe 24 making a close fit in the small bore 22 and extending out into the chamber. The probe 24 is held in position in a gas tight clamp by means of an olive 26 which fits loosely in the counterbore 23, and a T-nut 28 which encircles the probe 24 and screws into the threaded bore 23 to clamp the probe 24 by distortion of the olive 26. The wiring 25 passes through a slightly wider bore in the diffuser 17 and through the probe 11 to the box 15.

This arrangement is particularly useful in the analysis of liquids with a fairly high setting temperature, for example melted lard. Once the liquid has set, not only are the pressure measurements meaningless, but they might upset the associated electronics outside the tank. Thus by taking temperature readings regularly, a setting of the contents can be determined, the last pressure reading stored and the pressure sensing equipment turned off until the temperature reading indicates that the pressure reading would again be useful (i.e.

that the contents are again liquid.) Moreover the knowledge of the temperature of the contents means that the pressure readings and data calculated therefrom can be more effectively analysed.

It will be understood that the unit could equally well be used with a coaxial pair of hydrostatic probes by fitting it into the end of the inner of the two probes.

Although it is possible to operate this system with a completely steel sheathed temperature probe, use of a flexible sheathed probe such as a PT 100 type platinum resistance element is preferable. This probe has a ceramic element sheathed in metal, the wiring for which is simply sheathed in polytetrafluorethylene. This means that the wiring can be cut to the appropriate length and fitted to the terminals in the box 15 by the system fitter.

To assemble the combined sensor, firstly the temperature sensor probe is fitted into the unit as described above. The wiring for the probe is then threaded through the tube 11 and the unit is tack welded to the end of the probe 11. At the top of the tube 11 the wiring is cut to the appropriate length and attached to the terminals in the box 15.

Figure 3 illustrates a unit which can replace the unit of Figure 1 and in all respects other than those described hereinafter is the same as that of Figure 1. In this embodiment the tube 11 has bubble holes 31 spaced around the circumference of the bottom of the tube so it is unnecessary for the unit to incorporate a diffuser. Therefore there are no diffuser channels 19. The unit is of the same diameter as the tube 11 so facilitating the fitting procedure. As with the unit of Figure 1 the shield 18 is welded to the body 21 and the unit is welded to the end of the hydrostatic tube 11.

Figure 4 is a unit for attachment above the bottom of a hydrostatic probe. In large tanks the probe is formed from sections of tubing joined together by a mating of respective male and female ends screw threaded and welded together. The unit of Figure 4 can be slotted between any two such sections and for that purpose comprises a male threaded portion 32 at one end and a female threaded portion 33 at the other end. The unit is constructed of solid heat-conductive material, normally metal, and at the upper end is counter-bored to receive hydrostatic probe gas and electrical wires for the temperature sensor. The bottom of the counterbore 34 leads to two passageways, the first passageway 35 leads through the unit and provides a conduit for the hydrostatic probe gas through the unit. The second passageway 36 leads to an enclosed chamber 37 housing the temperature sensor 38.The electrical wires 39 lead through the passageway 25 to the main tube 11 through which they pass to the terminal box 15 as described with reference to Figure 2.

The temperature sensor 38 is held in the unit in direct contact with the material of the unit body by means of a grub screw 41 which passes through the unit from the outside and engages and bites the sensor urging it against the side of the chamber 37.

In a separate embodiment which is not illustrated, the chamber 37 is open to the tank contents and the sealed attachment of the temperature sensor 38 is through a nut and olive attachment as described with reference to Figure 1 and as illustrated in Figure 3.

With a unit that can be fitted into the tube above the bottom of the tank it is possible to overcome the problem of temperature gradient at the bottom of the tank.

If the temperature is taken above the normally solid contents a reasonable representation of the temperature of the liquid contents is obtained. This can be as high as 10 different from the temperature at the bottom of the tank.

It should be noted that in most embodiments the diameter of the hydrostatic pressure measurement system is not increased by the addition of the temperature unit. This means that an assembly of temperature and pressure sensors can fit through the same hole that would have been required for the temperature sensor alone. In the embodiment of Figure 1 the unit is wider that the tube 11, but if coaxial pressure probes were used the diameter of the unit would not be greater than that of the outer probe. it will be noted that the passageways 35 and 36 are angled to the vertical. This does have the advantage that the sensor is brought nearer to the outside of the unit which will make the temperature sensing more accurate during fluctuations in temperature.

The most appropriate material for the unit is 316 stainless steel.

Claims (12)

1. A temperature unit for attachment to a hydrostatic pressure probe and containing a temperature sensor connected to associated electrical wiring, the unit comprising means for supporting the temperature sensor, in use, in heat exchange relationship with the contents of the tank but out of the gas flow associated with the hydrostatic probe.

2. A temperature unit according to claim 1 wherein the unit includes a chamber in which the temperature sensor is housed.

3. A temperature unit according to claim 2 wherein the chamber is in flow communication with the tank contents.

4. A temperature unit according to any of the preceding claims including a conduit for hydrostatic probe gas.

5. A temperature unit according to claim 2 or 3 wherein the chamber additionally forms a spacer for separating the end of the hydrostatic probe from the bottom of the tank.

6. A temperature unit according to claim 4 or 5 wherein the conduit forms a diffuser portion which allows gas to exit from the hydrostatic probe.

7. A temperature unit according to claim 1 or 2 wherein the heat exchange relationship is indirect, the unit including a heat conductive material between the sensor and the tank contents so that the temperature of the material adjacent the sensor is substantially the same as that of the contents of the tank.

8. A temperature unit according to claim 4 or 7 wherein the unit comprises means for attachment between two sections of hydrostatic probe tubing and the said conduit passes through the unit to convey the gas from one to the other of the said two sections.

9. A temperature unit for attachment to a hydrostatic pressure probe substantially as herein described with reference to Figures 1,2, and 3 oS the accompanying drawings.

10. A temperature unit for attachment to a hydrostatic pressure probe substaially as herein described with reference to Figure 4 of the accompanying drawings.

11. A combined temperature and pressure measuring system for a tank comprising a hydrostatic pressure probe, a unit in accordance with any of the preceding claims attached to the probe, the electrical wiring from the temperature sensor passing up the probe, and a terminal connected to the probe to which the electrical wires are connected.

12. A system according to claim 11 wherein the unit fits into the bottom end of a section of hydrostatic probe tubing and the overall diameter of the pressure measuring system is not increased by the addition of the unit.