Valves Mixproof valve systems

valves

Mixproof valve systems

There are many junctions in a piping system where product normally flows from one line to the other, but which must sometimes be closed off so that two different media can flow through the two lines without being mixed. When the lines are isolated from each other, any leakage must go to drain without any possibility of one medium being mixed with the other.

This is a common problem faced when engineering dairy plants. Dairy products and cleaning solutions flow in separate lines, and have to be kept separate. Figure 6.8.5 shows four different solutions to the same task.

Shut-off and change-over valves

There are many places in a piping system where it must be possible to stop the flow or divert it to another line. These functions are performed by valves.

Seat valves, manually or pneumatically controlled, or butterfly valves, are used for this purpose.

Seat valves

The valve body has a seat for the closing plug at the end of the stem. The plug is lifted from and lowered on to the seat by the stem, which is moved by a crank or a pneumatic actuator, figure 6.8.6.

The seat valve is also available in a change-over version. This valve has three to five ports. When the plug is lowered the liquid flows from inlet 2 to outlet 1, and when the plug is lifted to the upper seat, the flow is directed through outlet 3, according to the drawings to the right in figure 6.8.7.

This type of valve can have up to five ports. The number is determined by the process requirements.

Various remote controlled actuator alternatives are available. For example, the valve can be opened by compressed air and closed with a spring, or vice versa. It can also be both opened and closed by compressed air, figure 6.8.8.

Actuators for an intermediate plug position and for two-stage opening and closing are also available.

The valve control unit, figure 6.8.9, is often fitted as a unit on the top of the valve actuator. This top unit usually contains indication sensors for the valve position for feedback to the main control system.

A solenoid valve is fitted in the air conduit to the valve actuator or in the top unit. An electric signal triggers the solenoid valve and allows compressed air to enter the actuator. The valve then opens or closes as required. On the way, the compressed air passes through a filter to free it from oil and other foreign matter that might affect proper operation of the valve. The air supply is cut off when the solenoid is de-energized and the air in the product valve is then evacuated through an exhaust port in the solenoid valve

Butterfly valves

The butterfly valve, figure 6.8.10, is a shut-off valve. Two valves must be used to obtain a change-over function.

Butterfly valves are often used for sensitive products, such as yoghurt and other cultured milk products, as the restriction through the valve is very small, resulting in very low pressure drop and no turbulence. It is also good for high viscosities and, being a straight-through valve, it can be fitted in straight pipes.

The valve usually consists of two identical halves with a seal ring clamped between them. A streamlined disc is fitted in the centre of the
valve. It is usually supported by bushes to prevent the stem from seizing against the valve bodies.

With the disc in the open position, the valve offers very low flow resistance. In the closed position the disc seals against the seal ring.

Manual control

The butterfly valve is fitted with a handle, usually for two positions open and closed.

This type of valve is not really suitable as a control valve, but can be used for coarse control with a special handle for infinite positions.

Automatic control

An air actuator, figure 6.8.11, is used for automatic control of the butterfly valve. The function can be:

• Spring closing/air opening (Normally closed, NC)
• Air closing/spring opening (Normally open, NO)
• Air opening and closing (A/A)

The disc is easy to turn until it touches the seal ring. Then it needs more power to compress the rubber. A normal, spring powered actuator is strongest in the beginning, when less power is required, and weaker at the end, when more power is required. It is therefore an advantage to use actuators which are designed so that they provide the correct power at the right
time.

Another type of the butterfly valve is the sandwich valve shown in figure 6.8.12. It is the same type of butterfly valve as described above, but it is fitted between two flanges welded to the line. Its function is the same as an ordinary butterfly valve. During operation it is clamped between the flanges with screws. For servicing the screws are loosened. The valve part can then be pulled out for easy servicing.

Mixproof valves

Mixproof valves, figure 6.8.14, can be either double or single seated, but when discussing mixproof valves, it is generally the double-seat type, figure 6.8.13, that is meant.

A double-seated valve has two independent seals separating the two liquids and a drainage chamber between. This chamber must be open to atmosphere to ensure full mixproof safety in case either of the two seals should leak. When a double-seated mixproof valve is activated, the chamber between the upper and lower body is closed and then the valve opens to connect the upper and lower pipelines. When the valve is closed, first the upper plug seals and then the leakage chamber is opened to atmosphere. This gives very small product losses during operation.

An important thing is that the lower plug should be hydraulically balanced to prevent pressure shocks from opening the valve and allowing products to mix.

During cleaning one of the plugs lifts, or an external CIP line is connected to the leakage chamber. Some valves can be connected to an external cleaning source for cleaning those parts of the plugs which have been in contact with the product.

The single-seat mixproof valve has one seat and two seals, but on the same plug. The area between the two seals is open to atmosphere. This leakage drain chamber is closed by small shut-off valves before the singleseat mixproof valve is activated. An external CIP line is connected to the drainage line via the small valves for cleaning.

Position indication and control

Position indication only

A valve can be fitted with various types of position indication, see figure 6.8.15, depending on the control system of the plant. Different types of switches are microswitches, inductive proximity switches or Hall elements. The switches are used for feedback signals to the control system.

When only switches are fitted to the valves, it is necessary to have one solenoid valve for each valve in a solenoid-valve cabinet on the wall. A solenoid valve supplies compressed air to the product valve when it receives a signal and releases the air pressure when the signal disappears.

This system (1) requires one electric cable and one air hose for each valve.

The combined unit (2) is usually fitted on top of the valve actuator. It contains the same types of position indicators as above, but the solenoid valve is also built into the top. This means that one air hose can supply many valves, but one electric cable per valve is still required.

The ultimate control

This is effected by a position indicating unit, shown in figure 6.8.9, which is specially designed for computer control. It contains position indicator, solenoid valve and an electronic unit. With this unit it is possible to control up to 120 valves with only one cable and one air hose, figure 6.8.15, ref. 3. A unit like this can be programmed centrally, and the installation costs are low.

Some systems can also, without external signals, flip valves for seat cleaning. They can also count the number of valve strokes. This can be used for maintenace planning.

Check valves

A check valve, figure 6.8.16, is fitted when it is necessary to prevent the product from flowing in the wrong direction. The valve is kept open by the liquid flow in the correct direction. If the flow stops, the valve plug is forced against its seat by the spring. The valve then closes against reversal of the flow.

Control valves

Shut-off and change-over valves have distinct positions, open or closed. In the regulating valve the passage can be changed gradually. The control valve is used for accurate control of flows and pressures at various points in the system.

A pressure relief valve, figure 6.8.17, maintains the pressure in the system. If the pressure is low, the spring holds the plug against the seat.When the pressure has reached a certain value, the force on the plug overcomes the spring force and the valve opens. The opening pressure can be set to the required level by adjusting the spring tension.

Manual control valve with variable-flow plug, figure 6.8.18.This valve has a stem with a specially shaped plug. When the regulating handle is turned, the plug moves up or down, varying the passage and thereby the flow rate or the pressure. A scale on the valve indicates the setting.

The pneumatic control valve with variable-flow plug, figure 6.8.19, works similarly to the previously described valve. The plug-and-seat arrangement is similar to that of the manual valve. The flow is gradually throttled when the plug is lowered towards the seat.

This type of valve is used for automatic control of pressures, flows and levels in processes. A transmitter is fitted in the process line and continuously transmits the measured value to a controller. This controller then adjusts the setting of the valve so that the preset value is maintained.

A valve often used is the constant-pressure valve, figure 6.8.20. Compressed air is supplied through a reducing valve to the space above a diaphragm. The air pressure is adjusted by the reducing valve until the product pressure gauge shows the required pressure. The preset pressure is then maintained regardless of changes in the operating conditions. Figure 6.8.21describes the function of the constant-pressure valve.

The valve reacts rapidly to changes in the product pressure. A reduced product pressure results in a greater force on the diaphragm from the air pressure, which remains constant. The valve plug then moves downwards with the diaphragm, the flow is reduced and the product pressure increased to the preset value.

An increased product pressure results in a force on the diaphragm that is greater than the downward force from the compressed air. The valve plug then moves upwards, increasing the passage for the product. The
flow will then increase until the product pressure has dropped to the preset value. This valve is available in two versions for constant pressure before or after the valve.

The valve cannot control the product pressure if the available air pressure is lower than the required
product pressure. In such cases a booster can be fitted to the top of the valve. In this way the valve can be used for product pressures up to about twice the available air pressure.

Valves for constant inlet pressure are often used after separators and pasteurisers. Those for constant outlet pressure are used before filling machines.

Valve systems

Valves are arranged in clusters to minimise dead ends and make it possible to distribute the product between different parts or blocks within the dairy. Valves are also used to isolate individual lines so that one line can be safely cleaned while the product is flowing in others.

Pipe supports

Pipes usually run about 2 3 metres above the dairy floor. All components must be easily accessible for inspection and maintenance. The lines should slope slightly (1:200 1:1000) to be self-draining. There should be no pockets at any point along the line where the product or cleaning fluid can collect.