US3672225A

US3672225A - Gas sampling

Info

Publication number: US3672225A
Application number: US63528A
Authority: US
Inventors: Rodney Brian Louis
Original assignee: African Explosives and Chemical Industries Ltd
Current assignee: African Explosives and Chemical Industries Ltd
Priority date: 1969-08-15
Filing date: 1970-08-13
Publication date: 1972-06-27
Anticipated expiration: 1989-06-27
Also published as: DE2040593B2; GB1267885A; ZA695850B; DE2040593A1

Abstract

Apparatus for sampling fluid flow in a conduit comprising a sampling probe having an aerodynamically shaped nozzle and a double static tube axially located within the nozzle and extending beyond its mouth to terminate in a closed, aerodynamically contoured end, the outer wall of the extended portion of the tube being provided with first and second orifices to communicate with the bore of the tube and the annular passage of the tube remote from the extended portion operatively communicating with a pressure sensing device to sense the static pressure of the fluid flow outside the nozzle and the static pressure of the fluid flow entering the mouth of the nozzle.

Description

United States Patent Louis 1 June 27, 1972 54] GAS SAMPLING 2,452,224 10/1948 Collett, Jr

[72] Inventor: Rodney Brian Louis, Johannesburg, Re-

33:52: et pub! Smth Afnca 3,050,996 8/1962 Henderson ..73/182 [73] Assignee: African Explosives and Chemical Industries Limited Primary Examiner-Louis R. Prince Assistant Examinerwilliam A. Henry, II [22] Flled' 1970 Attorney-Cushman, Darby and Cushman Appl. No.: 63,528

[57] ABSTRACT Apparatus for sampling fluid flow in a conduit comprising a sampling probe having an aerodynamically shaped nozzle and a double static tube axially located within the nozzle and extending beyond its mouth to terminate in a closed, aerodynamically contoured end, the outer wall of the extended portion of the tube being provided with first and second orifices to communicate with the bore of the tube and the annular passage of the tube remote from the extended portion operatively communicating with a pressure sensing device to sense the static pressure of the fluid flow outside the nozzle and the static pressure of the fluid flow entering the mouth of the nozzle.

5 Claims, 2 Drawing Figures P R ESSURE SENSING PATENTEDJum I972 3.672.225

EEEEJ' P'A'TENTEDJum 1972 3. 672,225

sum 2 OF 2 FIGURE 2.

/l diw GAS SAMPLING This invention relates to apparatus for sampling fluid such as dust or mist-laden effluent gases from chemical or industrial plants, and particularly to an isokinetic sampling probe used in sampling these gases.

Sampling probes used for determining the dust or mist in stack gases often produce unreliable results or are difficult to handle and, in view of present legislation covering atmospheric pollution in many countries, hinder the routine analyses of effluent gases which have to be carried outin view of this legislation. It is important, therefore, that apparatus be of simple construction and be easy to manipulate.

The conventional apparatus for sampling effluent gases comprises a sampling probe having a shaped nozzle, which is inserted into the stream to be sampled. The gas is drawn by means of a vacuum pump through the probe and a sampling train consisting of a filter, a cyclone or electrostatic precipitator, followed by a condenser, catch-pot and a rotameter.

' The particulate matter is trapped in the filter and water vapor, if present, is condensed in the condenser.

The gas sample must be collected under isokinetic conditions, that is, the velocity of the gas entering the nozzle of the probe should be the same as the velocity of the gas stream from which the sample is taken, otherwise the sample is not truly representative of the effluent stream. To ensure these conditions, the velocity of the gas stream must be continuously monitored and, when deviations occur, the sampling rate has to be adjusted accordingly. This is normally done by using Pitot tubes and the readings obtained are converted to velocities by means of slide rules or nomographs supplied with the apparatus. At the end of the sampling period, the weight of particulate matter collected in the filter is determined and related to volume of gas sampled to obtain the contaminate concentration in the stack gas.

An improved apparatus for sampling fluid uses a so-called Null Point Probe, that is, a probe having a specially designed nozzle through which the static pressures inside and outside the nozzle can be measured by means of orifices situated in the inner and in the outer walls of the nozzle construction. By maintaining these inside and outside pressures equal, isokinetic conditions are obtained. The use of these nozzles obviates the measurement individually of the velocity of the gas stream and of the sampling rate. Nevertheless, it has been found that the static pressures measured via the sensing orifices on the inside and outside of the nozzle are influenced by the nozzle shape. The shape of the nozzle causes eddy and vortex formations on the outside, as well as a pressure loss on the inside of the nozzle. Consequently, erratic results are obtained when using these nozzles.

A further disadvantage is that the nozzles are not readily interchangeable so that, at high or low gas velocities, the sampling rates are either very high or extremely low respectively.

It is the principal purpose of the present invention to provide a null point sampling probe, which largely or wholly eliminates the difficulties encountered in the prior art.

We have now discovered that these undesirable influences afiecting static pressures because of the shape of the nozzle may be avoided by sensing the static pressures outside, and at the entrance to, the nozzle. This advance in the art is achieved by means of a double tube comprising two concentric tubes which provide an axial bore and an annular passage surrounding the wall of that bore. This new feature, which is introduced as part of a null point probe, we designate by the expression double static tube.

Accordingly, we provide apparatus for sampling fluid flow in a conduit comprising a sampling probe having an aerodynamically shaped nozzle and a double static tube as defined herein axially located within the nozzle and extending beyond its mouth to terminate in a closed, aerodynarnically contoured end, the outer wall of the extended portion of the tube being provided with first and second orifices to communicate with the bore and with the annular passage respectively of the tube and the opposite end of the bore and the annular passage of the tube remote from the extended portion operatively communicating with a pressure sensing device to sense the static pressure of the fluid flow outside the nozzle and the static pressure of the fluid flow entering the mouth of the nozzle.

The first orifices of the double static tube for sensing the pressure outside the nozzle are located, preferably, at a distance of at least 13 tube diameters from the streamlined end of the tube to avoid any eddy or vortex formation within the fluid, which may be caused during its flow past the end of the tube.

The second orifices for sensing the pressure of the fluid flow entering the nozzle are located, preferably, on or adjacent the outside of the plane of the mouth of the nozzle. This position has been determined to avoid a pressure drop within the nozzle. Additionally, these second orifices should be at a distance of at least 21 tube diameters from the streamlined end of the tube to avoid possible interference caused by the first orifices.

The pressure sensing orifices in the tube have a diameter, preferably, of not more than one tenth of the external diameter of the tube.

The nozzle, which is part of the probe, may be replaced without difliculty, since the pressure sensing devices are not located in the body of the nozzle.

An embodiment of the apparatus will be described with reference to the accompanying drawings, in which FIG. 1 is a cross section of the assembled sampling probe, and

FIG. 2 is an enlarged cross section of the double static tube, forming part of the probe.

In FIG. 1, the sampling probe l is shown with detachable nozzle 2 and a double static tube 3. The first pressure sensing orifices 4 and the second pressure sensing orifices 5 in the wall surrounding the central passage of the extended portion of the double static tube 3 are in communication with a pressure sensing device via manifold 6 and tubes 7.

FIG. 2 illustrates the detailed construction of double static tube 3 and shows the location of the pressure sensing orifices 4 and 5 in relation to the streamlined end 9 of the tube. The frontal portion 8 of the tube may be made of solid material.

The apparatus is used in conjunction with a standard filter, cyclone or electrostatic precipitator and a vacuum pump. Any sensitive sensing device may be used to measure the pressure difference between the first and second sensing orifices.

The size of the nozzle used is dependent on the velocity of the gas stream and, preferably, is selected to give a sampling flow rate of between 5 I and 6 cubic meters per hour under isokinetic conditions, so that the sampling periods for collecting a representative sample are between 5 and 10 minutes.

Water vapor, if present in the gas flow, may condense in the cold parts of the sampling probe and a heated mantle 10 may be provided around these parts to prevent this condensation.

The probe was tested in an experimental duct with sampling velocities ranging from 5 to 25 meters per second and four sizes of nozzles were used to cover this range.

The sampling flow rate was measured on an accurate rotameter and the difference between this measured flow rate and the flow rate calculated from the velocity and the nozzle means defining a closed, aerodynamically contoured end;

said double static tube including means defining a longitudinal bore terminating short of said closed, aerodynamically contoured end;

a means defining a first orifice communicating the exterior of the double static tube, toward said end from said mouth, with said bore;

means defining an annular passage in said double static tube surrounding said bore, but being separated therefrom;

means defining a second orifice communicating the exterior of the double static tube, toward said end from said mouth, with said annular passage;

said first and second orifices being axially displaced from one another along said double static tube;

a pressure sensing device for sensing the static pressure of the fluid flow outside the nozzle and the static pressure of the fluid flow entering the mouth of the nozzle;

conduit means independently communicating said bore and said annular passage to said pressure sensing device remotely of and upstream of the mouth of the nozzle as inputs to said pressure sensing device.

2. Apparatus as claimed in claim 1 in which the first orifice for sensing the static pressure outside the nozzle is located at a distance of at least 13 tube diameters from the end of the tube.

3. Apparatus as claimed in claim 1 in which the second orifice for sensing the static pressure inside the nozzle is located no further upstream than a radial plane across the mouth of the nozzle at a distance of at least 2! tube diameters from the end of the tube 4. Apparatus as claimed in claim 1 in which each of the pressure sensing orifices in the tube has a diameter of not more than one tenth of the external diameter of the tube.

5. Apparatus as claimed in claim 1 in which the nozzle of the probe is detachably connected thereto, so as to be interchangeable with other noules of different dimensions and shapes.

* t F l

Claims

1. Apparatus for sampling fluid flow in a conduit, comprising: a sampling probe, including means defining a tubular nozzle having means defining a mouth; a double static tube extending axially within said nozzle and extending out past the mouth thereof to terminate in means defining a closed, aerodynamically contoured end; said double static tube including means defining a longitudinal bore terminating short of said closed, aerodynamically contoured end; a means defining a first orifice communicating the exterior of the double static tube, toward said end from said mouth, with said bore; means defining an annular passage in said double static tube surrounding said bore, but being separated therefrom; means defining a second orifice communicating the exterior of the double static tube, toward said end from said mouth, with said annular passage; said first and second orifices being axially displaced from one another along said double static tube; a pressure sensing device for sensing the static pressure of the fluid flow outside the nozzle and the static pressure of the fluid flow entering the mouth of the nozzle; conduit means independently communicating said bore and said annular passage to said pressure sensing device remotely of and upstream of the mouth of the nozzle as inputs to said pressure sensing device.

3. Apparatus as claimed in claim 1 in which the second orifice for sensing the static pressure inside the nozzle is located no further upstream than a radial plane across the mouth of the nozzle at a distance of at least 21 tube diameters from the end of the tube.

4. Apparatus as claimed in claim 1 in which each of the pressure sensing orifices in the tube has a diameter of not more than one tenth of the external diameter of the tube.

5. Apparatus as claimed in claim 1 in which the nozzle of the probe is detachably connecTed thereto, so as to be interchangeable with other nozzles of different dimensions and shapes.