Critical security systems diaphragm valve failure analysis and prevention
Diaphragm Valve structure and operating principle
Steam turbine adopts digital electro-hydraulic control system, in which hydraulic diaphragm valves (hereinafter referred to as diaphragm valves) are the key components contact control oil and security asphalt and lubricants emergency shutoff asphalt, when manual gate or the host excessive speed, lubricating oil system Emergency shutoff asphalt, the diaphragm valve upper oil pressure also leaks, stem rapidly move up, making the control system security oil connect with return oil and then pressure relief, automatic main valve and adjust valve fast closure, the crew shut down.
Diaphragm Valves are FISHER products, produced by Canada. The main components are the valve head (upper bonnet, diaphragm, down bonnet, spring tray, spring), stem, adjustable nut and body. Rated working pressure of diaphragm is 0.69 ~ 0.75 MPa, after the upper diaphragm establish oil pressure, the diaphragm downward movement, at the same time of overcoming spring stretch and promoting the stem down, makes the valve closed; when the upper diaphragm loses pressure, spring rebound to drive stem upward, the valve opens quickly.
Failure
(1) The bonnet sealing surface leakage. Diaphragm Valves head is composed by the upper and lower bonnet and diaphragm uses bolt to fix, the cavity between upper bonnet and the diapgragm inlets host lubricating oil system emergency trip oil, this sealing surface from the generating units putting in the production, continue to leak, repeated treatment results are ineffective.
(2) diaphragm rupture causes oil pressure loss, diaphragm valves opens. Diaphragm valve diaphragm rupture, lubricating oil flow to the high temperature pipes, so had to stop and replace the new diaphragm. Later found that there is a 3 mm perforation in the contacting part of diaphragm and spring tray.
reason analysis
(1) adjusting oil pressure adjustment door (for pressure relief) is the needle valve, due to vibration and other factors, during the operation it will gradually shut down, causing oil pressure rising slowly, if the adjustment is not timely, it will make the diaphragm operate under the 0.80 ~ 0.85 MPa ultra-pressure, accelerated the diaphragm damage. In addition when the host was doing excessive speed test, the rotating number rises to 3300 r / min or so, due to adjustment is not in time, diaphragm valve oil pressure had risen to 0.9 MPa, causing short-term overpressure.
(2) the upper and lower bonnet sealing surface design width is too small. Through examination, we found that the whole effective sealing width of leak diaphragm (extrusion deformation part) is less than 5 mm, the narrowest part is only 2 mm.
(3) the upper and lower bonnet plate thickness is just 5 mm, according to the upper bonnet effective diameter 240 mm, oil pressure 0.8 MPa to calculate, the bearing pressure is about 36 170 N, bonnet plate deforms due to the force, the upper and lower bonnet sealing plane generated inner port, so makes the sealing surface further narrow, leakage increased.
(4) diaphragm installation process is unreasonable. Maintenance personnel are not familiar with the structural principle of diaphragm valves, when in the installation of a new diaphragm, fails to take the measures of loose adjustable nut to make the spring loses force, but under the state of diaphragm valve is opening. As the spring tray at high position, diagram contacts with the upper bonnet, but exists 20 mm gap with the lower bonnet. In order to fasten the three together, it is necessary to use the long-rod bolts forced to connect (with the aim to overcome the spring stretch), this time the diaphragm bolt hole deforms due to pulling, easily lead to damage, and its bolt hole location moves inside relative the bonnet bolt hole, sealing surface becomes narrow and sealing effect is worse.
(5) the diaphragm glial part has many air bubbles, internal part has stratification phenomenon, while the maintenance staff checked careless before the replacement, failed to discover on time, so there exists failure hidden danger after installation.
(6) diaphragm accidental damage. In storage, installation process, you do not properly protect diaphragm, diaphragm contacts with corrosive liquids or with tools, sharp part of spare parts(for example, there is spring tray raised edges) collision, compression, damage, scratch the diaphragm surface, failed to deal with, resulting in leakage hidden danger.
Precautions
(1) Avoid diaphragm overpressure operation. Operating personnel should step up surveillance and find oil pressure is excessive, shall immediately notify the maintenance personnel checking pressure gauges. After the confirmation of increased oil pressure, should slowly rotate and close the adjustable door, will stabilize the pressure within rated range. Particularly attention to in the unit speed up, and the host overspeed test, the designated officers are responsible for monitoring oil pressure and timely adjusting.
(2) The transformation of the upper and lower bonnet. Through demonstration and experiments, while under the circumstances of maintaining control oil system security unchanged and not affect the diaphragm valves features, imitating the original parts design form to new processing bonnet, will increase the thickness to 10 mm, the width of the sealing surface inward extension to 10 mm. This measure is respectively taken into practice in 1,2 machine in 2003,2004, the effect is perfect and with no leak so far.
(3) improve the diaphragm installating process. Before replacing diaphragm, firstly measuring, recording adjustable nut exposed size, would spiral it downward, makes the spring free, spring-tray at the lowest position. Now install the diaphragm, no more space exists, can be symmetrical fastening bolts evenly. After installation, will spira the adjustable nut upwardl into its original position.
(4) careful inspection, to eliminate using substandard products. Diaphragm should be kept properly stored; when in the installation process, it is necessary to pay attention not to make the diaphragm struck, scratched; careful to check the upper bonnet, spring tray, to eliminate glitches and maintain contact area smooth. Through aboved treatment, diaphragm valve works well, ensuring the long-term safe operation of the main equipment.
