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Abnormal pressure in the valve chamber hazards and protection

From:Net   Date:2011-6-7
Abnormal pressure in the valve chamber hazards and protection
Abstract: The common gate abnormal pressure in the cavity formation mechanism and its harmfulness, presented abnormal pressure balance chamber design or installation of utility protection program for all types of users reference selection WZ40H-40 bellows valve .

Keywords: gate; abnormal pressure; pressure relief valve; relief hole; Outer Bypass


Valve is usually a single gate, double plate, wedge gate valve, parallel structures such as design, in which wedge double plate (Z42, Z62 series) and flexible wedge gate (Z40, Z60 series) the most widely used, the former Close fit on both sides of the valve seat when the gate automatically, automatically compensates for the processing of the wedge angle position error; Z41 cast steel gate valve which relied on the flexibility of the central groove, the axial thrust compensation by the stem wedge angle machining position error, both to achieve better sealing effect. Precisely because of its excellent bilateral sealed, in some cases produce abnormal rise of pressure in the chamber, i.e., when high temperature high pressure fluid (liquid or gas) in the cavity of the valve is blocked, if the upstream side (see Figure 1 ) fluid temperature in the chamber (see Figure 1) the heat transfer fluid can be synchronized elevated off the product can not expand in the cavity, the cavity sealing fluid from the cold state to a hot state, the liquid may be rapidly vaporized, resulting in pressure increased sharply, PZ673 pneumatic knife gate valve is often elevated pressure geometrically.


Figure 1 illustrate abnormal pressure in the cavity

Overpressure valves work very serious consequences. Z43H / X steel plate gate valve when an abnormal pressure in the cavity, the pressure parts and the opening and closing parts of the working stress (such as the use of stem and gate frame stress) will dramatically increase the driving force of the driving mechanism will be unbearable burden, can not even start to pull off serious stem, gate frame breakage, motor burned out, these phenomena in many large-diameter high-pressure valve in common, many users often complain that this is the gate "killed" in fact "bite death "the real reason is often abnormal pressure in the cavity of the" invisible killer "no diversion hole plate valve.

Typical case such as Z962Y Series Wedge Gate Valve used in thermal power plant water supply system and bypass, the type of valve usually first in the cold state for the hydrostatic test off when the unit starts when the system temperature rises to 250 ~ 300 ℃, the pressure valve as temperatures soar, the closed central cold water temperature will rise sharply synchronized vaporization of the fluid volume increases, the pressure increases, this time to open the valve to either drive torque is large enough, or stem assembly intensity is high enough, or often appear stem fracture, fracture gate frame, gate T-slot flowering, fracture, so that the feed pump fails to start, resulting in serious downtime Z673X shutdown pneumatic slurry valves.

Two abnormal pressure hazards

The formation of abnormal pressure valve in many applications will occur strikingly similar, as it occurs in many industrial systems the two elements are similar, the system boot medium, heat up the cold state; shutter in a cold state closed, open when hot. Thus if the system measures the destruction of abnormal pressure on the system is almost impossible to avoid, there are three aspects of its hazards CYZ41H oil seal differential pressure valve:

(1) the destruction of the valve itself,

Valve body, bonnet and stem parts of the valve nominal strength generally designed its strength, abnormal pressure, its opening pressure is doubled, resulting in the use of stress related parts increased exponentially, the actual stress when the material exceed the allowable stress, high stress areas will produce fracture failure, manual slurry valve causes the valve can not be opened, the valve will damage the machine or scrapped.

(2) the destruction of the system security

Obviously, housing, and other pressure parts bonnet overpressure is very dangerous, once its weak parts overpressure may perforation occurs first, causing the media leaks; their fill and seal parts tend to be from the high-pressure fluid out , causing a lot of medium leakage. When the medium is high temperature gases, toxic gases, toxic gases will be more severe, and even cause damage to equipment and personnel bevel gear slurry valves.

(3) the production control process causing huge losses

Normal opening and closing of the valve is the key to all kinds of industrial process control, once this control can not be achieved, the system should crash downtime, which will cause a huge loss, directly or indirectly, the next shot wedge gate valve.

3 protective measures

From the design, installation, commissioning and other aspects, virtually eliminating abnormal pressure in the cavity is entirely possible induction program used three sets of reference for the majority of users:

(1) inside the valve opening pressure relief hole

Solve the fundamental abnormal pressure in the cavity is to balance the pressure in the cavity, the creation of relief hole is the most cost effective solution, Z41T iron gate shown in Figure 2, respectively on the upstream side of the gate and the inlet side of the pressure relief valve opened cylindrical hole, which cavity pressure increases, the pressure in the cavity will automatically discharge to the upstream side, and always maintain pressure in the chamber is equal to the upstream pressure, thus avoiding the occurrence of abnormal pressure.


Figure 2 inside the valve opening pressure relief hole

(2) Install the bypass relief valve

Have been shipped outside of the valve can be installed and peripheral bypass relief valve reduces pressure in the chamber, flapper valve is shown in Figure 3.


Figure 3 bypass pressure relief valve is installed outside

Using peripheral bypass valve with China Unicom in the cavity and the upstream side of the gate is closed when the main shut-off valve can be turned off (depending on temperature changes in the cavity, is too high to be turned on). When you open the main gate, you should first open the bypass valve to reduce pressure in the chamber and then start the main gate.

(3) special valves externally mounted pressure relief valve


Figure 4 valves install special relief valve

Figure 4 shows the valve in the cavity in the peripheral pressure relief valve, set the discharge pressure relief valve rated working pressure of the main valve, which chamber overpressure, automatic discharge to the set pressure, so as to maintain the main the safe operation of the electric gate slurry valves.


Figure 5 structural design of pressure relief valve

Figure 5 is a pressure relief valve structure design (for reference). Pressure relief valve installed before the cut-off valve, pressure relief valve for easy adjustment and maintenance. Pressure relief valve pressure setting is usually considered according 1.33PN (PN for the system valve nominal stress) Manual Wafer knife gate valve.

In addition, the valve debugging, especially electric valve debugging, you should pay attention to the closing stroke and torque control, torque transfer small as possible will turn off some, death to prevent wedge gate; temperature valve stem should be considered a high temperature thermal expansion wedge gate caused death, the proposed high temperature and pressure when debugging large diameter valve, gate valve stem back in place properly and avoid real bite accidents.

4 Conclusion

Overpressure valve is a two-seat class invisible killer, sprocket-type slurry valve is a serious threat to the safe operation of equipment and systems, we must pay attention; valve, especially high temperature, high pressure, large diameter valve design and installation must be considered abnormal ultra- the occurrence of pressure and take the necessary precautions.

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