Gate valve is the key component of opening and closing fluid in wellhead assembly and oil production tree equipment. It has large quantity and many specifications. Its performance is critical to safety and reliability. Once the valve fails, drilling or production of light oil (gas) wells will stop, resulting in huge economic losses, and casualties may occur in serious cases.

China's demand and export of petroleum equipment is still in the increasing stage, but the domestic and foreign users'requirements for product safety and reliability continue to improve, and the demand for products with higher performance level is increasing. For example, the valves provided by Shell Company must satisfy PR2 requirements, and some western oil and gas fields and offshore oil fields in China have also put forward PR2 performance requirements for their products. Whether the design and manufacture of valves meet the performance requirements of PR2 level must be verified by a series of performance appraisal test procedures specified in API6A standard.

1. Requirements for performance appraisal test of PR2 class of valves.

Valves should first be tested according to API6A (ISO10423)/GB22513 (hereinafter referred to as "standard") and then PR2 performance test according to the standard. Detailed test procedures are as follows:

(1) 160 open and close cycle tests under pressure at room temperature;

(2) 20 open and close cycle tests at the highest temperature;

(3) 20 open and close cycle tests with pressure at the lowest temperature;

(4) The temperature cycle of the valve body and the high and low pressure holding tests at various stages;

(5) Torque test of valve in each test process.

2. Valve design parameters and design features

2.1 Design parameters

Preliminary Discussion on the Performance Appraisal Test of High Pressure Valve in Grade PR2

2.2 Design Structural Characteristics of Manual Flat Gate Valve

2.2.1 Important structure as shown in Figure 1

Preliminary Discussion on the Performance Appraisal Test of High Pressure Valve in Grade PR2

1 body, 2 gate, 3 stem, 4 stem nut, 5 cover, 6 diaphragm, 7 packing cover, 8 handwheel, 9 stem sleeve

10 pins, 11 bearing caps, 12 stem fillers, 13 sealing rings, 14 seats, 15 seats, 16 seats

Figure 1 Manual Flat Gate Valve

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2.2.2 Structural Characteristics

A. The gate valve adopts single gate, single seat and concealed rod structure.

B. Thrust bearing design is adopted at the stem sleeve to reduce the torque.

C. Floating design of gate and seat;

D. Valve two-way sealing design, service life extension;

E. Metal tapered sealing rings are used for body cover sealing.

F. Seat and gate are metal sealed;

G. Seat and body are sealed by metal and energy storage ring.

H. The valve stem adopts reverse sealing design, which can replace the stem packing with pressure.

2.3 Differences between PR2 and PR1 gate valves in design and performance appraisal tests:

PR1 performance test, standard mandatory three cycle tests at room temperature, other high and low temperature tests can only provide objective evidence, obviously, PR2 valve requirements are much higher than PR1. The design structure of PR2 class valve is similar to that of PR1 class valve. The differences are as follows:

(1) The smoothness of all sealing surfaces is improved by one grade;

(2) All flaw detection of all sealing surfaces has been improved to PSL3 level acceptance.

(3) Seat seal adopts two inner and outer energy storage rings (PR1 has only one energy storage ring);

(4) The appearance of gate and valve seat is hardened by spraying carbide (PR1 includes nitriding, surfacing, spraying and spraying, etc.).

(5) All thrust bearings adopt needle-rolling thrust bearings with strong bearing capacity (PR1 stage adopts ball-rolling thrust bearings).

Analysis of PR2 Performance Appraisal Test of 3 Valves

The performance appraisal tests of two valves with different design parameters were carried out by Jianghan Mechanics Research Institute, Zhongjianghan Institute of Quality Supervision and Inspection of Wellhead Equipment in National Oil and Gas Field. The tests did not include non-metallic seals. Among them, 2-1/16 "10000 Psi gate valves were tested twice. The first test failed due to leakage of packing during low temperature circulation. After rectification, PR2 test was carried out again and passed smoothly for the second time. 2-1/16 "15000Psi gate valve passes smoothly at one time.

3.1 PR2 test and rectification

The details of low temperature failure in the first test of 2-1/16 "10000 Psi gate valve are as follows: when the temperature is -30 degree C, the valve operates normally, but with the decrease of temperature, the leakage begins to increase; after disassembly and inspection, it is found that the stem packing is not damaged, but all seating rings are damaged, slight scratches on the sealing surface of gate and seat, and all other parts and components are found. Normal; but through the pressure test curve and records, it is found that the valve torque is too large, especially at low temperature, the torque is about 2 times the normal temperature to reach 221N M, exceeding the designed 200N M. During the second test, some key components were rectified, and the final test passed smoothly. After disassembly and inspection, the packing and sealing ring are not damaged, the gate and valve seat are not scratched, and all parts are abraded within the normal range. According to the first failure, the author has rectified the material selection of the design. The comparison of the design and material selection before and after the rectification is as follows:

Preliminary Discussion on the Performance Appraisal Test of High Pressure Valve in Grade PR2

A kind of

Figures 2 and 3 are excerpts from the records of the low temperature cycle test of 2-1/16 "15000psi gate valve". The design and material selection of the 2-1/16 "10000psi gate valve are fully adopted. The results of the PR2 performance appraisal test successfully verify the usefulness of the design and modification of the 2-1/16" 10000psi gate valve.

Preliminary Discussion on the Performance Appraisal Test of High Pressure Valve in Grade PR2

Fig. 2 Record Table of Wellhead Assembly and Oil Production Tree Performance Appraisal

Preliminary Discussion on the Performance Appraisal Test of High Pressure Valve in Grade PR2

Fig. 3 Real-time pressure chart of gate valve cryogenic cycle test

3.2 Analysis of the Reasons for the Success or Failure of the Test

Through the above three PR2 performance appraisal tests, the reasons for the failure of the test are basically found. Detailed topics and analysis are as follows:

(1) Low temperature leakage of valve stem seals is due to the failure of the selected seals to meet the minimum temperature requirements;

(2) Damage of Seat Seal Ring, indicating that Seat Seal Ring can not resist temperature change and tightness.