A comprehensive test of all technical performance indicators of the safety valve requires a full-performance test. The most basic requirement for the test is to have an actual operating condition, or even exceed it.
In addition to high-temperature and high-pressure vessels, this type of testing apparatus must be equipped with various fast-measuring instruments to ensure a large flow of high-temperature and high-pressure steam sources, which involves substantial expenses.
Calibration is part of the safety valve testing and is a major item in the factory’s pre-delivery tests.
Prior to leaving the factory, the safety valve is generally calibrated using a room temperature calibration bench for air medium opening pressure setting and seal testing.
The safety valve’s room temperature calibration bench can only be used for opening pressure adjustment and seal testing.
1. Scenarios when a safety valve needs calibration
1) Before first use or after a long period of storage.
2) Regular calibration.
3) Valves that are severely damaged or rusted.
4) Valves with missing nameplates.
5) Valves with damaged lead seals.
2. The Significance and Methods of Adjusting Reseat Pressure
The reseat pressure, as defined by national standards, refers to the static pressure at the valve inlet when the safety valve re-engages with the valve seat after reaching the discharge state.
This occurs when the medium pressure drops to a certain value and the opening height becomes zero. Both excessively high and low reseat pressures are undesirable; too low, it could result in loss of medium and energy, too high, it may not reach the discharge volume, leading to frequent valve bouncing.
The principle is to maximize the reseat pressure as much as possible while achieving the discharge volume, thereby reducing the loss of medium and energy.
Reseat pressure is adjusted through the adjustment ring. The principle of adjustment is based on the gap principle; the smaller the gap, the greater the resistance during ejection, the greater the force holding down the valve core, and the more difficult it is to reseat.
Conversely, the larger the gap, the easier it is for the valve core to fall back, and the higher the reseat pressure.
For safety valves with only a lower adjustment ring, raising the adjustment ring reduces the reseat pressure, while lowering the adjustment ring increases it.
For safety valves with upper and lower adjustment rings, reducing the distance between the adjustment rings lowers the reseat pressure, while increasing the distance raises it.
3. Verification Methods and Their Pros and Cons
Safety valve verification is conducted through two means: on-site verification (online verification) and test bench verification.
Where possible, on-site verification should be prioritized as it aligns more closely with actual operating conditions, hence, it’s more reliable.
(1) Pros and Cons of On-site Verification of Safety Valves:
Pros of on-site verification: It facilitates the verification of welded safety valves, allows for backseat pressure measurement, and provides accurate readings.
Cons of on-site verification: The verification process is time-consuming, the system requires repeated pressurization, it’s uneconomical, relatively dangerous, and seal integrity tests cannot be performed.
(2) Pros and Cons of Ambient Temperature Verification Bench for Safety Valves:
Pros: a. It resolves the setting and leak checking of safety valves for ambient temperature media and working temperatures below 250℃; b.
It establishes a small error range for the opening pressure of safety valves, saves adjustment time for new safety valves, reduces labor intensity, minimizes energy consumption, and reduces work hazards.
Cons: There’s a discrepancy between operating temperature and ambient temperature (springs soften at high temperatures), and it’s unable to verify backseat pressure.
(3) Media Used for Ambient Temperature Verification
Ambient temperature verification typically uses air, nitrogen, and water (for pressures above 20Mpa). Flammable, combustible, or toxic and harmful media such as oxygen, hydrogen, and acetylene cannot be used for verification.
Kerosene, gasoline, and diesel are also unsuitable as media sources. Although carbon dioxide is non-toxic and non-combustible, it can easily freeze and block the pipeline.
4. Principle of Safety Valve Verification
Pressurized medium is introduced into the inlet of the safety valve to be verified.
As the medium pressure rises to the point where the safety valve opens, the pressure at this moment is measured, representing the opening pressure, which is then adjusted to the specified value to complete the verification of the opening pressure.
Subsequently, as the pressure falls to the specified value (90% of the opening pressure), the absence or presence of medium leakage is checked either by observing the pressure gauge or using other statutory methods, thus verifying the seal integrity.
5. Repair of Safety Valves
The primary aspect of safety valve repair involves the valve sealing surfaces. Therefore, the material of these sealing surfaces must meet high standards.
The basic requirements are:
- it should possess high strength to withstand significant sealing pressure ratios;
- it should have sufficient impact toughness to endure the shock of the valve core quickly bouncing back into position;
- it should have high hardness to resist erosion from the medium;
- it should be rust-proof to prevent rapid leakage;
- it should be heat resistant to withstand high temperatures and remain effective with mediums like steam without softening;
- it should be corrosion-resistant to withstand acid and alkali erosion.
The main forms of hard sealing in safety valves include flat sealing, conical sealing, and spherical sealing, with flat sealing being the most commonly used.
(1) Repair of the Safety Valve Sealing Surface
The safety valve sealing surface comes in two types: metallic and non-metallic. Non-metallic seals are typically used for ambient temperature media, while metallic seals are often used for high-temperature, high-pressure media.
When the non-metallic seal surface is damaged, the sealing ring is usually removed and replaced. If it cannot be removed or there are no spare parts, turning machining is used.
For metallic seals, if the damage is not severe, grinding is usually employed, which includes rough grinding and precision grinding.
The valve seat and core must be ground separately using flat grinding tools; co-grinding is not allowed as it leads to grooves, causing leaks once the valve is opened and reseated.
(2) Grinding Tools and Materials
Tools: Grinding Flat Plate:
a. Cast iron (for rough grinding).
b. Carbide (for precision grinding).
Grinding head (for grinding the valve seat).
Abrasives: Chromium Oxide, White Corundum, 280#, 360#, 600#, W4, W2.5, W1.5.
Grinding Agents: Oleic Acid, Machine Oil, Vegetable Oil, Lard.
(3) Grinding of Sealing Surfaces
Grinding involves the use of abrasive tools and compounds, which, under certain pressure, slightly cut the work surface, removing an extremely thin layer of metal.
This process results in a workpiece surface with high flatness and a superior level of surface finish, a process known as grinding.
The principle of grinding: During the process, the abrasive compounds added to the grinding tools are partially embedded in the tool under the pressure exerted by the workpiece and the tool.
As the tool and workpiece perform complex relative movements, the abrasives slide and roll between the two, causing cutting and rubbing action, removing a layer of peaks from the surface of the workpiece.
Meanwhile, the abrasive compounds react chemically, swiftly forming an oxide layer.
6. Safety Valve Relevant Standards
Considering the significance of safety valves, numerous related standards and regulations have been developed worldwide.
In its early stages, China’s safety valve standards largely referred to those of the former Soviet Union.
The current safety valve standards in use include:
- GB/T12241 (General Requirements for Safety Valves)
- GB/T12242 (Safety Valve Performance Test Methods)
- GB/T12243 (Spring Direct Load Safety Valves)
- ZB-J98-013 (Technical Conditions for Power Station Safety Valves)
- API520/API526/API527 (American Standards)