Non-retum valves

All LC pumps require the use of a non-return valve and so the function of non-retum valves will now be described. 

Non-return valves are essential in the operation of pumps. It is important to ensure that when the pump compresses the solvent, it leaves at the exit port and is completely arrested at the inlet port. Conversely when the pump draws fresh solvent into the cylinder, the non-retum valves must allow solvent to flow through the inlet valve, but flow-back from the other side of the exit valve must be... 

Modem designs fitted with non-retum valves through which the plasticised rubber compound passes into the injection cylinder portion are capable of giving precise shot volume. The non-retum valve is activated by the injection pressure. The injection ram/non-retum valve unit ensures complete purging of the injection chamber. 

The rubber mix is extruded by a screw through a non-retum valve into a separate chamber from which it is injected into a mould by a simple ram set at an angle to the screw. The tight fitting separate piston gives accurate delivery of the rubber mix allowing more efficient application of ram pressures up to 160 MPa. This pressure can inject even unusually high viscosity natural rubber mixes of Mooney viscosity (ML1+3, 120 °C) up to 100. 

Excessive soak time can cause a problem if the material is heat sensitive. If extra caution is necessary, a heat probe can be inserted into the nozzle, or the endcap can be removed for checking the melt directly. Once the endcap is removed, insert a temperature probe inside the non-retum valve. The material within a non-retum valve is generally the last to reach operating temperature. The Auto-Shut valve can be checked by using pliers to gently pull open the poppet. 

New OEM machines come with soak timers. The function of the soak timer is to lock out the injection unit until the timer times out. The timer starts once the heaters reach the operator-set temperature. Our experiences with OEM soak timers are that they are satisfactory for many materials and front-end components. However, exercise caution if you are unfamiliar with the material, or your front-end components. Our belief is that many non-retum valves and materials require 40% more soak time than the OEM timer provides. 

The Auto-Shut Valve requires more soak time than conventional non-retum valves. This valve has a spring loaded poppet that rides in the body of the valve. There is a pool of plastic contained inside the valve by the poppet. The pool of plastic in the center of the valve, and the poppet shaft, are the last front end components to reach operating temperature. The plastic around the screw will reach operating temperature before the internals of the Auto-Shut Valve. If screw rotation occurs too soon, the plastic from the screw will ftow into the valve, and either the poppet will resist opening, or the cold slug of plastic in the valve will block the flow. If the screw continues to rotale, the Auto-Shut valve may become damaged. 

A non-retum valve fitted to the pump inlet will reduce this hazard. A Buchner flask fitted with a rubber stopper carrying a vacuum release stopcock and a glass tube reaching to the bottom of the flask will provide a means of preventing a back flow of water into the evacuated system. Should the back flow of water be insufficient to close the non-retum valve, then it will be seen to enter the Buchner flask. The stopcock can be opened very slightly, the resulting increase of pressure in the flask will... 

Resin and hardener flowrate Can vary as a result of worn pumps, viscosity changes with temperature, blockages or leaks in pipewoik and nozzles, sticking of non-retum valves, etc. Positive displacement, electromagnetic or coreolis flowmeters... 

The left side of Figure 3-8 shows the non-retum valve in the open position, in which melt can pass into the injection chamber. On the right, the non-retum... 

The non-retum valve-like the shut-off valve-cannot be used with heat sensitive materials, because it would cause undesirable accumulation and decomposition of these materials. 

Waste Heat Boilers, Steam Drum, and Superheater safety valves, level indicators, vents, fittings, non-retum valves, water gauge glass (should be visible from control room) position of blowdown vessel ... 

Air cooler, or electric heater, solenoid valves, non-retum valves, etc. 

An annex to NRC Generic Letter 89-10 (USNRC 1989) lists the most common deficiencies of motor valves. Many of them apply to air-operated valves and non-retum valves too, and are as follows ... 

In the plant experience, even cases of valves on welded lines are recorded where maintenance could not be performed because a relative displacement of the two parts of the valve along the welded pipe axis was required (for example, non-retum valves with a diagonal bolted joint on the valve body). 

A series of three compressed gas accumulators and borated water at low pressure (1.8 MPa, relative), each connected to one of the three cold legs of the primary system. The volume of each accumulator is of about 333 m, 250 m of which are occupied by borated water at 2000 ppm boron. These accumulators are normally isolated from the primary system by non-retum valves only, as for the intermediate pressure (4.2 MPa, relative) accumulators commonly installed in PWRs. The connection lines with the primary system are of small diameter (approximately 50 mm), sufficient to supply, in case of primary depressurization, a slow and durable injection of borated water (indicative duration in typical cases >10 hours). Borated water injection performs the double function of maintaining the reactor sub-critical in the long term and of refrigerating it by flooding and vaporization (feed and bleed). 

The immediately obvious hazard in a dry air distribution system is the entry of chlorine into the air lines during pressure reversals. Some plants use non-retum valves to protect against this. The reliability of these valves is questionable, and the authors recommend the use of automatic valves that close when the differential pressure across them reverses. Figure 11.51 illustrates this technique in a different application. These valves can be used in conjunction with non-retum valves when some redundancy is desired. Section 12.7.2 reviews some of the arrangements used to connect air and other utilities to process systems. 

C.J. Rauwendaal, Non-Retum Valve with Distributive and Dispersive Mixing Capability, 58 SPE ANTEC, Orlando, EL, 638 641 (2000)... 

An acetylene supply plant consists of a high-pressure, medium-pressure and possibly a low-pressure section. In addition to the pressure regulators, safety devices such as manual safety cylinder or bundle connectors, non-retum valves, automatic quick acting shut-off devices, flame arrestors and multifunctional safety devices have to be installed. 

Kontrollventil check valve, control valve, non-retum valve... 

Non-retum valves have the task of blocking the volume flow in one direction and allowing free-flow in the opposite direction. The blockage should provide completely leakproof sealing. Balls or cones are used primarily as sealing elements. 

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