Hard sealed ball valves are much more stringent than soft sealed ball valves in service conditions. High temperature, strong corrosion, dust, particles, mud and other media have caused considerable obstacles to the continuous use of hard sealed ball valves. In the process of using hard seal ball valve, the valve is often forced to be repaired due to failure caused by working conditions or human factors.

It is necessary and effective for valve industry workers and product users to understand the causes of valve failures and find ways to prevent them.

The technical researchers of our company specially analyze the problems of hard seal ball valves that commonly appear in the use of ball valves by market users. And expand the problems that should be paid attention to in the product design and accessory equipment of hard seal ball valve, and put forward the direction of improvement.

From the perspective of product use, the faults of hard sealed ball valves can be roughly divided into the following two categories: non-technical faults and technical faults. The text below mainly introduces the analysis and solutions of non-technical faults of hard sealed ball valves;

1 non technical fault

Non technical fault literally means that the product is not caused by technical problems, that is, it is not the internal structure of the valve or the operation of internal parts. Therefore, the failure of the valve is generally caused by external conditions.

1.1 wrong valve selection

When purchasing valves, many customers lack understanding of the working conditions of the products or are considering the cost. The selected valves cannot meet the requirements of working temperature, working pressure, medium corrosivity and erosivity under the working conditions, resulting in the failure of the valves immediately when they are put into use.

Therefore, in the process of model selection, the customer or the design institute must put the reliability of the product in the first place, and the damage caused by the failure of choosing cheap valves is not worth the loss. This is particularly important in the selection of hard seal ball valves under severe working conditions.

1.1.1 the material selection of some valve internals is inconsistent with the operating temperature

In hard sealed ball valves, the material selection of some internals is closely related to the working condition temperature or medium temperature, such as packing, sealing gasket, valve stem sheath, etc. there are generally two reasons for choosing hard sealed ball valves: one is that the working condition temperature is high, and the soft sealed valve seat can't bear it; the other is that the medium abrasion ability and erosion ability are strong, and the soft sealed valve seat is not hard enough. It can be seen that hard sealed ball valves are not equal to high-temperature ball valves. In some working conditions, Its use temperature is not necessarily very high.

When the medium temperature is higher than 200 ℃, graphite needs to be used as the filler (graphite has strong temperature resistance, and its performance basically does not change under high temperature). The sealing gasket is generally wrapped around the graphite ring with metal wire (sealed with graphite, shaped with metal wire), and the valve stem sheath should also be replaced with metal material.

When the medium temperature is lower than 200 ℃, synthetic plastic can be selected for packing, sealing gasket (including body cover sealing gasket, lower valve stem sealing gasket, middle flange sealing gasket, etc.) and valve stem sheath. This configuration is similar to the configuration of relevant accessories in soft seal ball valve.

Therefore, in the selection process of hard seal ball valve, the working condition temperature or medium temperature must be taken as one of the necessary factors to determine the selection direction. The wrong choice of internals may result in waste of cost or failure of the valve.

1.1.2 wrong material selection of cemented carbide surface sprayed on the sealing surface of ball seat

The key to the performance of hard sealed ball valve lies in the surface hardening treatment of steel ball and valve seat. In order to ensure metal to metal sealing, the sealing specific pressure between hard sealed ball seats is much greater than that of ordinary ball valves, which makes it easy for metal materials to bite or strain, both of which will directly lead to the failure of ball valves to work normally. At present, the most widely used and the best effect of ball seat surface hardening method is the spraying cemented carbide Technology (mainly nickel base alloy and chromium base alloy). The steel ball and valve seat spraying layer of hard sealed ball valve largely determines the service performance and life of the ball seat sealing surface. Therefore, in the actual complex working conditions, the cemented carbide layer should be selected according to the actual situation.

There are roughly two directions for the selection of cemented carbide layer: one is to pursue its hardness. High hardness alloy layer has strong wear resistance and erosion resistance, which is often used in the working conditions with small and hard solid particles (such as coal chemical industry and polysilicon industry); The second is to pursue its corrosion resistance. In some highly corrosive media, the corrosion-resistant alloy layer can effectively protect the internal substrate of the ball socket, thereby extending its service life. It is commonly used in highly corrosive media (such as petrochemical and paper industry). In the selection of materials for hard sealed ball valves, the material for spraying cemented carbide layer on the sealing surface of the ball seat is the top priority, and cemented carbide that does not match the working conditions and service medium conditions must not be selected.

1.2 type selection failure of accessories

The accessories of the valve include the driving device, the supporting device of the driving device, the support connecting the driving device, the connecting shaft and other accessories. The driving device of hard sealed ball valve can be divided into lever handle, steel pipe handle, turbine actuator, hand wheel clutch reducer (used to switch manual operation or automatic operation), automatic actuator (including pneumatic actuator, electric actuator, electro-hydraulic actuator, etc.) according to its power source.

For lever handle, steel pipe handle, gear mechanism and other driving devices that rely on manual operation, calculate the torque according to the working torque value of the product, and then select the appropriate length or specification. Of course, the torsional tensile strength of the material also needs to be considered.

The selection of automatic actuator is relatively complicated. Take pneumatic actuators for example. Pneumatic actuators are divided into single acting and double acting types. Double acting pneumatic actuator switches have the effect of spring force, and the torque is consistent. Single acting pneumatic actuators should overcome the effect of spring force when opening and close by spring force. In this way, the output torque is a range of torque value, which increases the difficulty of model selection. Generally, when installing pneumatic actuators, it is necessary to multiply a safety factor based on the working torque value of the valve, Prevent the actuator output torque from insufficient operating force due to range changes.

Generally, when leaving the factory, the actual working torque value cannot be accurately measured before the actual working condition is reached. The commonly used standard is to estimate the working torque value by multiplying the no-load torque value by a medium coefficient. For hard sealed ball valves, this medium coefficient should be selected according to different working conditions. If it is particle, dust, mud medium, this coefficient should be as large as possible, generally in the range of 1.5 ~ 2.

In addition to the selection of the actuator itself, its accessories should also be selected according to the actual situation, such as relevant solenoid valves, pressure reducing valves, valve position switches, etc. for example, a pneumatic actuator with a large air intake is equipped with a solenoid valve with a small air flow, so that the air intake of the pneumatic actuator will be very slow, the valve will be closed slowly, and the longer the sealing surface will be eroded, which is not conducive to the service life of the valve. Moreover, the remarkable advantage of the valve of automatic device lies in its fast opening and closing time. Many customers require that the opening and closing action be completed within N seconds, which puts forward higher requirements for the reasonable configuration of relevant accessories.

1.3 other non-technical faults

In addition to the non-technical failures caused by the above common type selection errors, in the actual use of hard sealed ball valves, due to the installation direction, steps, etc. are not in accordance with the specifications, there may also be use failures. Therefore, during the use of valves, the position of the connection should be connected in accordance with the instructions to ensure safety.