The Potential Faults,Cause and Trouble-shooting Methods in Thermal Power Plant
I. Overview
The thermal system of a power plant is an organic whole connected by thermal equipment, steam pipes and various accessories. In this organic whole, steam pipes and valves are not only an inseparable part of the production system, but also occupy an important position. Only by reasonably arranging various valves in the piping system to control the movement of the medium can the pipeline facilities give full play to its effectiveness so as to satisfy the needs of the production process and ensure the safety of the system.
At present, the units of power plants are developing in the direction of large capacity and high parameters. Power plant valves are also developing in the direction of high temperature and high pressure with the increase of medium parameters. With the increase of working pressure of the medium, the valves are constantly improving the sealing structure and adopting the new type of sealing material that improves the sealing performance; with the continuous improvement of the automation level of the generator set, the valve actuation has been improved, and it is developing towards the direction of convenient centralized control and remote control.
II. Valve selection
The safe and efficient operation of power plants is inseparable from the correct and reasonable selection of valves. The valves on the pipeline can be selected according to factors such as use, medium type and medium working parameters (pressure, temperature and flow). When selecting, the nominal pressure, nominal diameter, allowable working temperature and use range of the selected valve should be adapted to the nominal pressure, nominal diameter and medium type in the piping system where the valve is located. At the same time, the convenience of installation, operation, maintenance and overhaul should also be considered, as well as economically reasonable.
III．Common faults, causes and elimination methods
1.Fault: valve body or bonnet leaks
Potential cause:
(1)Poor pouring during material casting causes blisters or cracks, resulting in reduced mechanical strength.
(2)The valve body is cracked during repair welding.
Solution
(1) Polish the suspected cracks and etch them with 4% nitric acid solution. If there are cracks, they will show up.
(2) Polish the cracked part with a grinding wheel to remove the cracked metal layer; process the groove for repair welding, and perform repair welding on the valve body and bonnet made of alloy steel at 250℃～300℃ Preheat, put in asbestos dust and cool to room temperature naturally after welding.
2. Fault : Body-bonnet joint leaks
Cause
(1) Bolt tightening force is not enough or tight;
(2) The gasket is damaged;
(3) Uneven joint surface or groove cracks, etc.
Solution:
(1) The bolts should be tightly angled, the tightening force should be consistent, and the joint surface gap should be consistent;
(2) Replace the gasket;
(3) Disassemble and repair the joint surface again. If the groove crack on the joint surface is deep, the groove can be polished with a grinding wheel for repair welding. After the joint flange is repaired, the weld is higher than the original plane, and the weld must be turned flat to ensure the flatness of the flange surface.
3. Fault: the shaft damage, such as thread damage or bending
Cause:
(1) Improper operation by excessive force;
(2) The thread fit is too loose or too light;
(3)There are too many operations and the service life is too long;
Solution:
(1) Improve operation;
(2) When manufacturing spare parts, it is necessary to meet the tolerance requirements and select materials appropriately;
(3) Re-process and replace parts.
4. Fault: Valve seat leak
Cause:
(1)Improper selection of trim material.
(2)Corrosion and damage of valve trims
Solution:
(1) Select qualified valve core material according to the nature and temperature of the medium;
(2) Grind the seat or ball;
(3)Replace the valve that meets the requirements and install it in accordance with the flow direction of the medium.
5. Fault: Packing leakage
Cause:
(1) The packing is not compacted;
(2)The number of packing circles is not enough;
(3) Packing wear and ageing.
Solution:
(1)Tighten the gland.
(2)If necessary, replace packing
6. Fault: Leakage at the connection of valve and pipeline
Cause:
(1)Flange nuts are not tightened or tightened unevenly;
(2) There is damage or dirt on end sealing surface ;
(3) The gasket is invalid.
Solution:
(1) Tighten the nuts evenly.
(2) Repair the sealing surface or clean up dirt;
(3) Replace the gasket.
IV. Conclusion
In summary, a quick and correct judgment of the cause of valve defects, together with timely and correct methods to eliminate them, can provide a better guarantee for the safe, stable, economical, and efficient operation of power plant units.