GB1593908A

GB1593908A - Reference electrodes and measuring systems for detemining the amount of disolved oxygen in a liquid

Info

Publication number: GB1593908A
Application number: GB20765/78A
Authority: GB
Original assignee: Babcock and Wilcox Co
Current assignee: Babcock and Wilcox Co
Priority date: 1977-12-05
Filing date: 1978-05-19
Publication date: 1981-07-22
Also published as: LU79771A1; ATA427278A; NL175951C; DE2829665A1; ES478875A1; SE7806243L; NL7804263A; JPS5480189A; IT1103071B; ES472261A1; DE2829665B2; FR2410823A1; BE867652A; IL54732A; FR2410823B1; NL175951B; JPS5838746B2; SE438736B; CA1096940A; IT7809542A0

Abstract

The high-temperature reference electrode (10) is used in a measuring device for determining the oxygen content of water at very high temperature. At one end it has a sealed tube (12), preferably made of a palladium-silver alloy. The latter is pressurised with hydrogen from a feed line (18) and is loosely enclosed in a perforated sleeve (14). Through openings (16) in the sleeve, water enters the narrow gap between the tube (12) and the sleeve (14), is retained in the gap and is saturated with hydrogen, which diffuses through the wall of the tube (12). This provides a constant value which serves as a reference level for the measurements of oxygen content carried out together with a measuring electrode. The reference electrode is used in boiler construction, mainly in nuclear reactor boilers. <IMAGE>

Description

(54) REFERENCE ELECTRODES AND MEASURING SYSTEMS FOR DETERMINING THE AMOUNT OF DISSOLVED OXYGEN IN A LIQUID (71) We, THE BABCOCK & WIL COX COMPANY, a corporation organized and existing under the laws of the State of Delaware, United States of America, of 161 East 42nd Street, New York, New York 10017, United States of America, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to reference electrodes and to measuring systems for determining the amount of dissolved oxygen in a liquid.

There is presently a great need for high temperature reference electrodes which may be used in measuring systems for determining the percentage of dissolved oxygen in high temperature water in the approximate temperature range of 450"F to 600"F.

Boilermakers, among others, require an accurate measurement of the amount of oxygen or the oxidizing power of the solution in contact with the various corrodible metals comprising the boiler. As an example, light water reactor systems require extreme safety measures because of the potentially catastrophic damage that could be caused by the failure of such a nuclear reactor system. One way for failure to occur in such reactor systems is by way of corrosion of the pipes or tubes conducting high temperature water through the reactor system vessel or steam generator. It is known that corrosion occurs when the concentration of dissolved oxygen in the water of a light water reactor system exceeds 0.2 ppm.

Since this level is found in normal city water light water reactor systems use treated water having dissolved oxygen levels not to exceed 0.2 ppm for boiling water reactor systems and not to exceed 20 ppb for steam generators of pressurized water reactor systems.

Oxygen contaminated water may accidentally enter one of the reactor systems.

Therefore, a dissolved oxygen measuring system is required which can measure the exact amount of dissolved oxygen in the water of the light water reactor system so that the critical oxygen level may be accurately monitored and controlled.

One of the problems of providing such a dissolved oxygen measuring system has been unavailability of a reference electrode which would function at the temperatures, approximately 550"F, at which the water is maintained in the secondary side light water reactor systems. High temperature reference electrodes are known utilizing Silver Silver Chloride alloys. Such high temperature reference electrodes are satisfactory except in situations where a reducing atmosphere is present, such as is found in a pressurized water reactor system or a fossil reactor system. The hydrogen in the water in the presence of a reducing atmosphere causes the Silver-Silver Chloride material to break down and the reference electrode is no longer Silver-Silver Chloride but becomes through chemical reaction a different material. Thus, it may be seen that the known high temperature reference electrodes are capable of operating only in situations where an oxidizing atmosphere is present.

From the foregoing we can see that what was needed was a high temperature reference electrode which could operate at temperatures of approximately 550"F in a reducing atmosphere such as is found in attempting to measure the dissolved oxygen content in water on the secondary side of a pressurized water reactor system.

According to a first aspect of the invention there is provided a reference electrode comprising: a tube having a closed end and an open end and made of an alloy material which is permeable to hydrogen gas at least when the electrode is disposed in water which provides a reducing atmosphere and has a temperature substantially in the range of 400"F to 5500F; a sleeve having a series of openings along the length thereof and being affixed around said alloy tube to define a space between said alloy tube and said sleeve; and means for connecting the open end of said tube to a supply of hydrogen gas.

A high temperature hydrogen reference electrode embodying the invention and described hereinbelow is capable of operating in a reducing or oxidising atmosphere. The electrode is formed from a closed end tube of Palladium-Silver alloy pressurised on the inside, in use, with pure hydrogen gas. The alloy tube is loosely encased in a lightly perforated sleeve which allows liquid such as water to be trapped between the alloy tube and the sleeve where the water is saturated with hydrogen permeating through the wall of the alloy tube. The electrode embodying the invention acts as a hydrogen reference electrode and it will not break down in any environment because the reaction providing the referencing is hydrogen to hydrogen ions regardless of the environment. The electrode is thus not only capable of measuring the amount of oxygen in a solution when used with a measuring system but may actually be used to measure the oxidising power of the solution. That is, if there are some other oxidisers such as ferric ions, chromate ions, or other such similar ions the measuring system utilising the mentioned reference electrode would also respond to that type of environment.

The use of the reference electrode embodying the invention in an oxygen measuring system may be as follows. The reference electrode as well as a second electrode responding to the solution to be measured would both be immersed in the solution. The two electrodes would then be electrically connected to a measuring instrument such as a high impedance voltmeter or electrometer. Nickel has been found to be a suitable material for the measuring electrode. The Nickel behaves in an oxygen environment as a second order oxygen electrode and its potential rises when it is in a solution that has dissolved oxygen. If the solution does not have oxygen, the potential of the Nickel electrode falls and the difference between the Nickel electrode and the reference electrode comes close to zero.

According to a second aspect of the present invention there is provided a measuring system for determining the amount of dissolved oxygen in a liquid. the system comprising: a hydrogen reference electrode comprising a tube having a closed end and an open end and made of an alloy material which is permeable to hydrogen gas at least when the electrode is disposed in water which provides a reducing atmosphere and has a temperature substantially in the range of 400"F to 5500F, a sleeve having a series of openings, along the length thereof and being affixed around said alloy tube to define a space between said alloy tube and said sleeve, and means for connecting the open end of said tube to a supply of hydrogen gas; a measuring electrode; and electric indicator means connected between said reference electrode and said measuring electrode for indicating the potential difference between said reference electrode and said measuring electrode.

The invention will now be further described, by way of illustrative and nonlimiting example, with reference to the accompanying drawings, in which: Figure I is a perspective drawing of a high temperature reference electrode embodying the present invention; Figure 2 is a schematic drawing of the reference electrode of Figure 1 being utilised in an oxygen measuring system for determining the oxidising power of high temperature water; and Figure 3 is a graph of the potential difference exhibited between the reference electrode and a secondary electrode in the measuring system of Figure 2 when subjected to water having different levels of dissolved oxygen.

Turning now to the drawings, Figure 1 shows a reference electrode assembly 10 having a closed end tube 12 made from a 75coo Palladium-25% Silver alloy material.

This alloy has been found to have significant permeability of hydrogen while remaining very corrosive resistant to high temperature water. The alloy tube 12 is loosely encased in a sleeve 14 having a series of perforations or holes 16 along the entire length of the sleeve 14. The sleeve 14 is made of inert material such as polytetrafluoroethylene (PTFE) plastics material and is loosely heat shrunk onto the alloy tube 12. PTFE was chosen because of its ability to be heat shrunk onto the alloy tube 12 as well as its temperature resistance. PTFE will not deteriorate at temperatures below 600"F.

PTFE will act as a barrier allowing water to flow only through the holes 16. Various other inert materials would serve just as well for the sleeve 14. In situations where higher temperatures beyond 600"F were to be encountered or in high velocity flow situations where the PTFE sleeve may be ripped off by the force of the velocity, a metal sleeve of Stainless Steel, Silver, or Nickel may be used. The criteria for the choice of material would be its corrision resistance.

non-pollution of the water stream and nonpermeability to hydrogen. Holes would have to be drilled or punched into the metal sleeve material to allow water to be communicated through the sleeve.

The open end of the alloy tube 12 is connected to a nonelectrical conducting tube 18 which leads to a supply of pressurized hydrogen gas which pressurizes the alloy tube 12 and allows hydrogen to be permeated through the wall of the alloy tube 12.

The alloy tube 12 is mounted to a well known Conax electrical fitting 20 having a threaded portion 22 which may be sealably threaded into a wall of a pressure vessel enclosing pressurized liquid to allow the closed end portion of the alloy tube 12 to be located within the liquid to be sensed. The end of the reference electrode assembly 10 has a threaded portion 24 through which the hydrogen gas tube 18 is coupled to the open end of the alloy tube 12 by way of a compression nut 26. A setscrew 28 is threaded through an adapter 29 attached to a RULON (Registered Trade Mark) packing gland of the Conax fitting 20 so as to contact the wall of the alloy tube 12 and to provide an electrical signal pickup therefrom. The setscrew 28 also acts as a coupling maintaining the alloy tube 12 affixed to the Conax fitting 20 thereby preventing the alloy tube 12 from being blow out of the Conax fitting 20 in applications where the alloy tube 12 is sealably mounted in a pressurized vessel.

To prevent the electrical signal tapped from the setscrew 28 from being grounded to the wall of any container into which the Conax fitting 20 will be mounted, an electrically insulating RULON packing gland is mounted between the alloy tube 12 and the Conax fitting 20. The packing gland 30 is a filled material such as PTFE filled with Alumina Oxides and which is commercially available.

Turning now to Figures 2 and 3, it will be seen that the reference electrode 10 may be used with a second solid material Nickel Nickel Oxide electrode 32 to provide a voltage signal in a high impedance voltmeter or electrometer 34 which is electrically connected between the reference electrode 10 and the second electrode 32 by electrical lines 36. The voltage signal established on the high impedance voltmeter of the electrometer 34 will be proportional to the amount of dissolved oxygen in the fluid in which both the electrodes are immersed. The two electrodes form half cells in which the potential developed is related by the wellknown NERNST equation to the hydrogen ion activity in one cell and the oxygen ion activity in the other cell. As may be seen, both the reference electrode 10 and the second electrode 32 are sealably threaded through a wall 38 on the secondary side of a light water nuclear reactor system so as to be immersed in the flowing water on the secondary side of the reactor system. The water flow is from the Nickel-Nickel Oxide electrode 32 to the reference electrode 10.

The electrode 32 is placed upstream of the reference electrode to prevent hydrogen contamination of the actual measuring electrode with reference hydrogen. The distance between the two electrodes is not criticial and may be maintained up to a couple electrode lengths. For convenience, the electrodes could be in close proximity to each other.

The water inside of the wall 38 on the secondary side of the light water reactor system will be at a temperature substantially in the range of 400"F to 5500F and will be at a pressure of approximately 1200 psi. To maintain the permeability of the hydrogen gas out of the wall of the alloy tube 12, the hydrogen gas supply connected to the alloy tube 12 by the tube 18 is maintained at a pressure higher than the 1200 psi in the secondary side of the reactor system and is held at a 1300 psi pressure level.

As was mentioned earlier, the operation of the cell would be as follows. Water flow inside the wall 38 would allow water to be trapped between the alloy tube 12 and the sleeve 14 by virtue of the holes 16 in the sleeve 14. The water trapped there would be saturated with hydrogen due to the permeability of hydrogen gas through the tube wall 12. As such, the electrode 10 would provide a hydrogen reference where the ion activity is from hydrogen to hydrogen ions and which is a saturated constant forming a half-cell. The oxygen ion activity on the secondary Nickel-Nickel Oxide electrode 32 would then provide a second half-cell potential difference between the reference electrode 10 and the secondary electrode 32 dependent on the amount of dissolved oxygen in the water.

Turning to Figure 3, it will be seen that the potential difference between these two electrodes in millivolts when immersed in high purity water of a temperature substantially in the range of 400"F to 550"F and at 1200 psi will vary with the oxygen concentration in parts per million as indicated. The high slope linear nature of the curve in the 0.1 ppm to 10 ppm dissolved oxygen level makes this an ideal system for detecting a corrosive water level in the secondary side of the light water reactor system. The slight slope of the curve in the 0.01 ppm to 0.1 ppm level also allows measurement of dissolved oxygen concentration in steam generators of pressurized water reactors.

From the foregoing, it will be seen that the high temperature reference electrode can be used in measuring systems measuring the oxygen content in high temperature high pressure water.

Certain improvements and modifications will occur to those skilled in the art upon reading this specification. Clearly the basic concepts disclosed herein could just as easily be applied to both low temperature measuring systems as well as extremely high temperature measuring systems operating at temperatures in excess of 600"F. For such extremely high temperature applications different materials would have to be chosen for the sleeve member capable of withstanding the extremely high temperatures. It will be understood that such improvements and modifications, not included herein for the sake of conciseness and readability, are within the scope of the following claims.

WHAT WE CLAIM IS: 1. A reference electrode comprising: a tube having a closed end and an open end and made of an alloy material which is permeable to hydrogen gas at least when the electrode is disposed in water which provides a reducing atmosphere and has a temperature substantially in the range of 400"F to 550"F; a sleeve having a series of openings along the length thereof and being affixed around said alloy tube to define a space between said alloy tube and said sleeve; and means for connecting the open end of said tube to a supply of hydrogen gas.

2. A reference electrode according to claim 1, wherein said tube is formed from Palladium-Silver alloy material.

3. A reference electrode according to claim 2, wherein said tube is formed from 75% Palladium - 25% Silver alloy material.

4. A reference electrode according to claim 1, claim 2 or claim 3, wherein said sleeve is a PTFE material tube having a series of perforations therealong and being heat shrunk onto said alloy tube.

5. A reference electrode according to any one of claims 1 to 4, including a hydrogen gas source and wherein said connecting means includes a threaded plug fitting mounted to said alloy tube and having a compression fitting for connecting a connecting line between said hydrogen gas source and the open end of said alloy tube.

6. A reference electrode substantially as herein described with reference to Figure 1 of the accompanying drawings.

7. A measuring system for determining the amount of dissolved oxygen in a liquid, the system comprising: a hydrogen reference electrode comprising a tube having a closed end and an open end and made of an alloy material which is permeable to hydrogen gas at least when the electrode is disposed in water which provides a reducing atmosphere and has a temperature substantially in the range of 400"F to 550"F, a sleeve having a series of openings along the length thereof and being affixed around said alloy tube to define a space between said alloy tube and said sleeve, and means for connecting the open end of said tube to a supply of hydrogen gas; a measuring electrode; and electric indicator means connected between said reference electrode and said measuring electrode for indicating the potential difference between said reference electrode and said measuring electrode.

8. A measuring system according to claim 7, wherein said reference electrode and said measuring electrode are sealably mounted through the wall of a pressurised vessel to be in contact, in use, with water.

9. A measuring system according to claim 8, wherein said electrodes are in contact with said water and the temperature of the water is substantially in the range of 400"F to 5500F.

10. A measuring system as set forth in any one of claim 7 to 9, wherein said tube is formed from Palladium-Silver alloy material.

11. A measuring system as set forth in claim 10, wherein said tube is formed from 75% Palladium - 25% Silver alloy material.

12. A measuring system according to any one of claims 7 to 11, wherein said sleeve is a PTFE material tube having a series of perforations therealong and being heat shrunk onto said alloy tube.

13. A measuring system substantially as herein described with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. can be used in measuring systems measuring the oxygen content in high temperature high pressure water. Certain improvements and modifications will occur to those skilled in the art upon reading this specification. Clearly the basic concepts disclosed herein could just as easily be applied to both low temperature measuring systems as well as extremely high temperature measuring systems operating at temperatures in excess of 600"F. For such extremely high temperature applications different materials would have to be chosen for the sleeve member capable of withstanding the extremely high temperatures. It will be understood that such improvements and modifications, not included herein for the sake of conciseness and readability, are within the scope of the following claims. WHAT WE CLAIM IS:

1. A reference electrode comprising: a tube having a closed end and an open end and made of an alloy material which is permeable to hydrogen gas at least when the electrode is disposed in water which provides a reducing atmosphere and has a temperature substantially in the range of 400"F to 550"F; a sleeve having a series of openings along the length thereof and being affixed around said alloy tube to define a space between said alloy tube and said sleeve; and means for connecting the open end of said tube to a supply of hydrogen gas.