Questions About Globe Valves, Which Ones Drive You Crazy Most?

- Aug 23, 2019-





Material selection:

GS-C251.0619,WCB.WCC,LCB,LCC,LC3,4C,4D,WC6,C5,C12, C12A,CA15CF3,CF8(-196’C),CF3M,CF8M,CF8C,CD4MCu,CK3MC   uN,ZG200-400,ZG230-450,ZG270-500,2G40Cr1,ZG20CrM0.2G15C   r1Mo1V,ZG35Cr1MO.ZG16CrMOG(1Cr5Mo),ZG1Cr13,ZGOCr18Ni9 Ti.5A.KmTB   Cr26.4A




Resin sand casting, etc.

Globe valve is one of the most widely used valves in industry. It is especially suitable for steam, heat conducting oil, other gases, hot and cold water, and various solvents. It is the best sealing performance and longest service life of all manual switch valves. In the process of using, we often encounter some problems. Now let's summarize and analyze them, hoping to be helpful to everyone.

Internal Leakage and Immortality

The specific reasons and solutions are as follows:

1. Impurity plugging:

A. If high hardness impurities (such as welding slag) are blocked to the sealing surface, the sealing surface will be damaged and leaked while closing the valve vigorously.

Solution: Pipes need to be cleaned; filters are added to the pipes; stainless steel filter mesh is embedded at the inlet end of the valve, the filter mesh hole spacing is less than the width of the sealing surface, so as to avoid impurities destroying the sealing surface.

B. Some rust, oil or other dirt sticks to the sealing surface, causing the valve to not close tightly.

Solution: Use soft cloth to dip some kerosene to wipe the sealing surface. If it is very hard calcified, use sandpaper to polish it, and then wipe it clean with cloth.

Butterfly valve

2. Internal leakage is caused by erosion and cavitations.

When steam passes through the valve, it usually flows in turbulent form and has a high velocity. If the steam contains water (droplets), the scouring force is very large because the density of water is more than 300 times that of steam (calculated here according to 5barG pressure). Therefore, the sealing surface of the valve is often washed out of the line groove, like brushed by steel. The same trace.

When the valve is used in sewage outlet, high-pressure steam, humid steam and other conditions, it is more vulnerable to cavitation and erosion damage, and its life is shorter.

For example: A, boiler sewage valve: the more impurities, the harder, the faster erosion corrosion speed, the more easy to leak.

B. In the grain and oil industry, before the stripper and the granulator modulator in the feed industry, the valve often operates at a small opening, and the flow rate increases, so it is easy to leak.

C. Valves on Boiler Distribution Cylinder:

Solutions to reduce erosion are as follows:

A. The most important thing is to reduce the flow rate and pressure difference.

B. Clean up impurities.

C. Improve the dryness of steam and remove droplets.

D. Improving the design of valve structure to minimize positive erosion. The sealing surface of valve is made of Satellite alloy with the best erosion resistance.

For example, bellows globe valves adopt high-in-low-out flow direction and double-disc balancing structure. When the small disc is opened first, the steam flow rate will decrease. As the steam enters the lower end of the big disc through the small hole, that is, when the outlet end of the valve, the pressure difference becomes lower and lower. Therefore, compared with the traditional low-in and high-out structure, this design allows the life of the sealing surface. Longer. The adjustable bellows globe valve can adjust the flow rate manually by equal percentage. The curved surface shunt valve disc and the plane seal are adopted on the sealing surface. It can effectively protect the sealing surface from the damage of capitation, erosion and capitation, and greatly increase the service life and sealing performance.

3. Too frequent switches and the sealing surface is worn out, which leads to leakage.

If the ordinary 13Cr series material is used, without hardening treatment, the processing accuracy is low and its wear resistance is poor. Therefore, the sealing surface is prone to leakage due to the wear of multiple switches. Therefore, the sealing surface should be made of materials with excellent wear resistance, corrosion resistance, high hardness and thermal stability. For example, the sealing surface of bellows globe valve is made of Satellite alloy.

The erosion depth of Satellite is 0.00127MM, 2Cr13 seat is 0.0508MM and brass seat is 0.1016-0.2032MM. It can be calculated that the erosion resistance of Satellite is 40 times as much as that of 2Cr13, 80-160 times as much as that of brass, and the sealing life is much longer than that of brass. Satellite alloy is a kind of cobalt chromium tungsten alloy with high hardness, excellent erosion resistance, corrosion resistance, wear resistance and thermal stability.

4. The integral valve seat is easy to leak due to casting defects and welding defects.

Independent seats are used to ensure the material, welding performance and processing accuracy of the sealing surface, thus achieving the best sealing effect. Brands and major foreign brands use this structure, and almost all other domestic brands use integral seats.

5. Chemical corrosion and stress corrosion of medium cause leakage.

For some medium such as industrial desalted water of boiler system, there is weak acid-alkali corrosion. Therefore, stainless steel should be used for inner parts of valves, and Stellite alloy is the best choice for sealing surface.

6. Sealing surface processing accuracy is not high, no grinding, easy to be eroded and corroded.

The flatness, roughness and coaxiality of the sealing surface are guaranteed during processing, and the surface is ground into a mirror.

7. Why is packing sealed globe valve more likely to leak?

Even if there is impurity plugging, it may not necessarily destroy the sealing surface. The sealing surface will be destroyed only when it bears the overload closing force. Because the packing seal is difficult to switch, customers usually use the booster rod, so the lever moment is very large. The actual closing force acting on the sealing surface may be greater than the maximum closing force it can bear. The specific calculation is as follows:

stainless steel Check valve

For PN1 6 globe valves, the maximum pressure is 16*1.5=24bar. For DN100 valves, the maximum closing force on the sealing surface is F=P*S=24* (3.14*102/4)=1884 kgf. When operating the hand wheel, the closing moment on the sealing surface is 1884*D (hand wheel), but if using the additional lever, the closing moment is 1884* [D hand wheel]. Wheel + D booster] is far greater than the maximum bearing moment, and the sealing surface is also deformed, resulting in leakage.

It is recommended to use bellows cut-off valve. The switch is very easy and there is no need to use an afterburner.

8. Leakage calculation of globe valve internal leakage

If DN100's globe valve has 100 MM in seat diameter, 10 barg of steam in medium, and the globe valve leaks, mainly because the sealing surface is eroded or contaminated by impurities. We regard DN100's globe valve as many small holes leaking. Assuming there are 10 0.2 MM holes, the steam loss is as follows:

According to Napier equation, the flow calculation formula is deduced. In the formula of flow (kg/h) = 0.4d2 (P1+1), d = aperture (mm), P1 = gauge pressure (Barg), then the steam loss of 10 0.2mm orifices at 10 Bar. pressure is 10*0.4*0.22* (10+1) = 1.76kg/h. If the plant continues to operate, the loss will be caused by the leakage of the DN100 shutoff valve for one year. The steam volume is 1.76*24*365=15418 kg=15.418 T. According to the current unit price RMB 200 per ton of steam, the annual loss is 15.418*200=RMB3083. 

The price of purchasing a zero leakage brand bellows globe valve is less than RMB 3000, so the recovery period is less than one year.Therefore, it is very meaningful and urgent to solve the internal leakage problem of globe valve.

Leakage from stem

The specific reasons and solutions are as follows:

1. Packing Sealed Globe Valve, Leakage occurs when the padding of the switch is abraded many times. Use bellows to seal to ensure zero leakage.

2. Bad quality bellows globe valves are easy to break due to material, manufacturing process and design defects of bellows, leading to leakage.

At present, most manufacturers use U-shaped bellows made of single 304 stainless steel with short service life. Bellows globe valves use double or three layers of 316L stainless steel_-shaped bellows with a service life of more than 10,000 times.

3. The structural design defect of bellows assembly causes leakage.

At present, the bellows of most factories are welded together with the valve stem. When the operator operates the valve with the booster rod, once the force is excessive, the lower end of the bellows rotates with the valve stem, and the upper end is compressed by the middle flange. The bellows are twisted and deformed and instantly suffer great radial torque, because the wall of the bellows is very thin. (Generally, 0.2-0.5MM/layer), so the corresponding shear stress will be large enough to exceed its tensile strength and lead to the breaking of bellows. Sometimes the bellows do not burst immediately, but because the bellows are distorted or eccentrically deformed, the fatigue life and the ability to bear pressure of the bellows are greatly reduced, which will eventually lead to the rapid fatigue fracture of the bellows and leakage. Therefore, optimizing the structure of bellows components is the key to solve this problem.

The BELLOWS GLOBE VALVE optimizes the design of the structure, separates the bellows from the stem, and combines the bellows directly with the top cover and disc, which fundamentally and thoroughly solves the problem of easy rupture of the bellows and makes it impossible for the bellows to leak out again. The structural drawings are attached.

4. Welding defects cause leakage.

----------- Pores appear in the welds of bellows and caps, which leak once perforated. It is necessary to improve the welding quality and material of bellows.

5. The impact of water hammer leads to the rupture of bellows.

Some shut-off valves installed at the end of the pipeline and behind the trap often cause serious water hammer due to the accumulation or backflow of condensate, and destroy the bellows, so we should try to eliminate the water hammer.

6. The use of overpressure results in cracks or cracks in bellows.

Use within the maximum allowable pressure range.

7. Valves sealed with packing are used in very permeable media such as heat conducting oil, which leads to leakage.

8. There are impurities stuck between the corrugates. When forced compression, the bellows deform, which greatly shortens the service life of the bellows.

9. Leakage calculation of globe valve leakage

If DN100 packing cut-off valve has 10 barg pressure vapor, if the packing is worn out by 1 mm, that is to say, the clearance between packing and stem is 0.5 mm on one side, and the known hole diameter of valve cover is 35 mm, the leakage amount can be calculated: (V is the velocity of flow in the formula, P is the density).

Q = AVP * 3600 = [3.14 * (0.0352-0.0342)/4] * 10 * 5.63584 * 3600 = 11kg/h

If the plant continues to operate, the steam loss caused by the leakage of this DN100 packing globe valve is 11*24*365=96360kg=96.36T. According to the current unit price of 200 yuan per ton of steam, 19272 yuan is lost every year.

The switch is tight to operate.

1. Plunger valves are used: when the new valve is too tight, the switch is quite difficult, so it must be operated with an afterburner. When the switch is relatively easy, the valve begins to leak.

2. Packing sealing globe valve is used: it is a little easier than plunger valve, but it is still tight. Small valves can be operated by hand, and large valves must be operated by an afterburner.

Solution: Using bellows cut-off valve, the switch is super easy, even large valves can be operated easily by hand, without the need for an afterburner.

3. Large caliber globe valves (DN125 or above) are designed with conventional low-in and high-out flow direction. When the valves are closed, the axial thrust of the fluid on the disc needs to be overcome. This force can not be achieved by manual operation of the handwheel, and it must be operated with the help of an afterburner rod.

Solution: Large caliber globe valve is designed with high-in low-out flow direction and double disc balancing structure, which perfectly solves this problem.

The switch is too frequent every day, sometimes forgotten, or lazy to switch;

In view of this kind of working condition, it is suggested to select pneumatic bellows cut-off valve and automatic switch-off. As in the right picture

The valve is stuck and cannot be opened.

1. The valve stem is rusty: if lubrication can not be switched on and off, then only new valves can be replaced;

2. Broken copper nut: Replace the copper nut, but if the screw tooth is broken and the slipper is broken, the new valve can only be replaced.

Leakage at middle flange: If the gasket is broken, replace the gasket.

The hand-wheel is deformed and unscrewed.

This is usually accidentally damaged in transit, and can be replaced by hand-wheels.

The bellows are broken.

The whole set of bellows needs to be replaced.

The valve is rusty and the paint falls down one by one.

Normal cast iron or carbon steel valves should be baked after spraying paint. If the paint is sprayed first and then baked, generally speaking, the paint will not fall off, but will only be yellowing for a long time. If the paint is directly brushed and not baked, the paint will fall off.

The sealing surface was seriously corroded and the valve cores were all lost.

In this case, acid and alkali liquids (such as heating coil of CIP cleaning system) are usually entered into the pipeline (if the coil is broken, alkali leaks into the coil, which will cause subsequent globe valves and traps to be corroded, resulting in leakage, even perforation and other hazards), or acid and alkali gases are mixed into the medium, causing serious corrosion. Stainless steel valves should be selected. Resin valves should be selected if they are strong acid and alkali.

Valve body perforation:

Generally speaking, if the valve body material is cast iron or ductile iron, because of sand casting, loose metallographic structure, easy to appear sand holes, may be perforated after a period of time. It is suggested to use cast steel, which is made by precision casting. Its metallographic structure is compact, durable, and perforation rarely occurs.

Disc shedding:

The disc of most manufacturers is positioned on the stem with a pin. Once the positioning pin is broken, the disc immediately falls off and the valve cannot be used.