How to control vacuum pressure

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Fundamentals of pressure monitoring, control, and regulation in vacuum systems

In all vacuum processes the pressure in the system must be constantly checked and, if necessary, regulated. Modern plant control additionally requires that all measured values which are important for monitoring a plant are transmitted to central stations, monitoring and control centers and compiled in a clear manner. Pressure changes are frequently recorded over time by recording equipment. This means that additional demands are placed on vacuum gauges:

a) continuous indication of measured values, analog and digital as far as possible
b) clear and convenient reading of the measured values
c) recorder output to connect a recording instrument or control or regulation equipment
d) built-in Digital interface (e.g. RS 232)
e) facility for triggering switching operations through built-in trigger points

These demands are generally met by all vacuum gauges that have an electric measured value display, with the exception of Mechanical Diaphragm and liquid-filled vacuum gauges. The respective control units are equipped with recorder outputs that supply continuous voltages between 0 and 10 V, depending on the pressure reading on the meter scale, so that the pressure values can be recorded over time by means of a recording instrument. If a pressure switching unit is connected to the recorder output of the gauge, switching operations can be triggered when the values go over or below specified setpoints. The setpoints or switch threshold values for triggering switching operations directly in the gauges are called trigger values. Apart from vacuum gauges, there are diaphragm pressure switches that trigger a switching operation (without display of a measured value) via a contact amplifier when a certain pressure is reached. Valves, for example, can also be controlled through such switching operations.

Automatic protection, monitoring and control of vacuum systems

Protection of a vacuum system against malfunctions is extremely important. In the event of failure, very high material values may be at risk, whether through loss of the entire system or major components of it, due to loss of the batch of material to be processed or due to further production down time. Adequate operational control and protection should therefore be provided for, particularly in the case of large production plants. The individual factors to be taken into account in this connection are best illustrated on the basis of an example: Fig. 3.20 shows the schematic diagram of a high vacuum pump system. The vessel (11) can be evacuated by means of a Roots pump (14) or a diffusion pump (15), both of which operate in conjunction with the backing pump (1). The Roots pump is used in the medium vacuum range and the diffusion pump in the high vacuum range (you could also use a Turbo molecular pump). The valves (3), (8) and (16) are operated electropneumatically. The individual components are actuated from a control panel with pushbuttons.

Fig 3.20 Schematic diagram of a high vacuum pump system with optional operation of a Roots

pump or a diffusion pump.

Backing pump
Backing pressure monitoring device
Electropneumatic valve
Compressed air connection
Pressure monitoring device
Temperature monitoring device
Cooling water monitoring device
Electropneumatic valve
Recorder
High-vacuum monitoring device
Vessel
High-vacuum gauge
Limit switches
Roots pump
Diffusion pump
Electropneumatic valve
Venting valve

Measures to protect the pump system against malfunction

The pump system is to be protected against the malfunctions as described below. The measures to be taken in order to forestall such malfunctions are also given:

a) Measures in the event of power failure: All valves are closed so as to prevent admission of air to the vacuum vessel and protect the diffusion pump against damage.

b) Protection in the event of a drop in pressure in the compressed air network: The compressed air is monitored by a pressure monitoring device (5). If the pressure falls under a specified value, a signal can initially be emitted or the valves can be automatically closed. In this case, a sufficient reserve supply of compressed air is necessary (not shown in Fig. 3.20), which allows all valves to be actuated at least once.

c) Measures in the event of failure of cooling water to the diffusion pump: The cooling water is monitored by a flow or temperature monitoring device (6) and (7). If the flow of cooling water is inadequate, the heater of the diffusion pump is switched off and a signal is given; the valve (8) closes.

d) Protection against failure of the diffusion pump heater: Interruption of the diffusion pump heating system can be monitored by a relay. If the temperature rises above a maximum permissible value, a temperature monitoring device (6) responds. In both cases the valve (8) closes and a signal is given.

e) Protection in the event of backing pump failure: Belt-driven backing pumps must have a centrifugal switch which shuts down the entire system in the event of belt breakage or another malfunction. Monoblock pumps for which the drive is mounted directly on the shaft can be monitored by current relays and the like.

f) Protection against a pressure rise in the vessel above a certain limit value: The high vacuum monitoring device (10) emits a signal when a specified pressure is exceeded.

g) Ensuring the critical forepressure of the diffusion pump: When a certain backing pressure is exceeded, all valves are closed by the backing pressure monitoring device (2), the pumps are switched off and again a signal is given. The position of the valves (3), (8) and (16) is indicated on the control panel by means of limit switches (13). The pressure in the vessel is measured with a high vacuum gauge (12) and recorded with a recorder (9). Protection against operating errors can be provided by interlocking the individual switches so that they can only be actuated in a predetermined sequence. The diffusion pump, for example, may not be switched on when the backing pump is not running or the required backing pressure is not maintained or the cooling water circulation is not functioning.

Pressure regulation and control in rough and medium vacuum systems

Control and regulation have the function of giving a physical variable – in this case the pressure in the vacuum system – a certain value. The common feature is the actuator which changes the energy supply to the physical variable and thus the variable itself. Control refers to influencing a system or unit through commands. In this case the actuator and hence the actual value of the physical variable is changed directly with a manipulated variable. Example: Actuation of a valve by means of a pressure-dependent switch. The actual value may change in an undesirable way due to additional external influences. The controlled unit cannot react to the control unit. For this reason, control systems are said to have an open operating sequence. In the case of regulation, the actual value of the physical variable is constantly compared to the specified setpoint and regulated if there is any deviation so that it completely approximates the setpoint as far as possible. For all practical purposes regulation always requires control. The main difference is the controller in which the setpoint and the actual value are compared. The totality of all elements involved in the control process forms the control circuit. The terms and characteristic variables for describing control processes are stipulated in DIN 19226.

Generally a distinction is made between discontinuous control (e.g. twostep or three-step control) with specification of a pressure window, within which the pressure may vary, and continuous control (e.g. PID control) with a specified pressure setpoint, which should be maintained as precisely as possible. We have two possible ways of adjusting the pressure in a vacuum system: first, by changing the pumping speed (altering the speed of the pump or throttling by closing a valve); second, through admission of gas (opening a valve). This results in a total of 4 procedures.

Discontinuous pressure regulation

Although continuous regulation undoubtedly represents the more elegant procedure, in many cases two-step or three-step regulation is fully adequate in all vacuum ranges. To specify the pressure window, two or three variable, pressure-dependent switch contacts are necessary. It does not matter here whether the switch contacts are installed in a gauge with display or in a downstream unit or whether it is a pressure switch without display. Fig. 3.21 illustrates the difference between two-step regulation through pumping speed throttling, two-point regulation through gas admission and three-point regulation through a combination of pumping speed throttling and gas admission. Figures 3.22 and 3.23 show the circuit and structure of the two two-step regulation systems. In the case of two-step regulation through pumping speed throttling (Fig. 3.22), voltage is supplied to pump valve 4, i.e. it is open when the relay contacts are in the release condition. At a level below the upper switching point the valve remains open because of the self-holding function of the auxiliary relay. Only at a level below the lower switching point is the relay latching released. If the pressure subsequently rises, the valve is opened again at the upper switching point.

Fig 3.21 Schematic diagram of two-step and three-step regulation

Fig 3.22 Two-step regulation through pumping speed throttling.

➀ Gauge with two switching points

➁ Throttle valve
➂ Vacuum pump
➃ Pump valve
➄ Vacuum vessel

Fu - Fuse
R, Mp - Mains connection 220 V/50 Hz
Smax - Switching point for maximum value
Smin - Switching point for minimum value
PV - Pump valve
R1 - Auxiliary relay for pump valve
K1 - Relay contact of R1
M - Measuring and switching device

Fig 3.23 Two-step regulation through gas admission

➀ Gauge with two switching points
➁ Variable-leak valve
➂ Inlet valve
➃ Gas supply
➄ Throttle valve
➅ Vacuum pump
➆ Vacuum vessel

Fu - Fuse
R, Mp - Mains connection 220 V/50 Hz
Smax - Switching point for maximum value
Smin - Switching point for minimum value
EV - Inlet valve
R2 - Auxiliary relay for inlet valve
K2 - Relay contact of R2
M - Measuring and switching device

In the case of two-step regulation through gas admission, the inlet valve is initially closed. If the upper pressure switching point is not reached, nothing changes; only when the pressure falls below the lower switching point, do the “make contacts” open the gas inlet valve and actuate the auxiliary relay with self-holding function simultaneously. Return to the idle state with closing of the gas inlet valve is not effected until after the upper switching point is exceeded due to the release of the relay self-holding function.

Fig. 3.24 shows the corresponding three-step regulation system which was created with the two components just described. As the name indicates, two switching points, the lower switching point of the regulation system through pumping speed throttling and the upper switching point of the gas inlet regulation system, were combined.

Fig 3.24 Three-step regulation system.

➀ Gauge with three switching points
➁ Variable-leak valve
➂ Variable-leak valve
➃ Inlet valve
➄ Gas supply
➅ Throttle valve
➆ Vacuum pump
➇ Pump valve
➈ Vacuum vessel

Fu - Fuse
R, Mp - Mains connection 220 V/50 Hz
Smax - Switching point for maximum value
Smitte - Switching point for mean value
Smin - Switching point for minimum value
T – GRPAHIX THREE
PV - Pump valve
EV - Inlet valve
R1 - Auxiliary relay for pump interval
R2 - Auxiliary relay for inlet interval
K1 - Relay contact of R1
K2 - Relay contact of R2
M - Measuring and switching device

To avoid the complicated installation with auxiliary relays, many units offer a facility for changing the type of function of the built-in trigger values via software. Initially one can choose between individual switching points (or “level triggers”) and interlinked switching points (“interval triggers”). These functions are explained in Fig. 3.25. With interval triggers one can also select the size of the hysteresis and the type of setpoint specification, i.e. either fixed setting in the unit or specification through an external voltage, e.g. from 0 – 10 volts. A three-step regulation system (without auxiliary relay), for example, can be set up with the Leybold CEREVAC and GRAPHIX THREE.

Fig 3.25 Diagram of level triggers and interval triggers

GRAPHIX – Operating Units for Active Sensors GRAPHIX Display and operating instruments for active sensors

Continuous pressure regulation

We have to make a distinction here between electric controllers (e.g. PID controllers) with a proportional valve as actuator and mechanical diaphragm controllers. In a regulation system with electric controllers the coordination between controller and actuator (piezoelectric gas inlet valve, inlet valve with motor drive, butterfly control valve, throttle valve) is difficult because of the very different boundary conditions (volume of the vessel, effective pumping speed at the vessel, pressure control range). Such control circuits tend to vibrate easily when process malfunctions occur. It is virtually impossible to specify generally valid standard values.

Many control problems can be better solved with a diaphragm controller. The function of the diaphragm controller (see Fig. 3.27) can be easily derived from that of a diaphragm vacuum gauge: the blunt end of a tube or pipe is either closed off by means of an elastic rubber diaphragm (for reference pressure > process pressure) or released (for reference pressure < process pressure) so that in the latter case, a connection is established between the process side and the vacuum pump. This elegant and more or less “automatic” regulation system has excellent control characteristics (see Fig. 3.28).

Fig 3.27 Principle of a diaphragm controller

Reference chamber
Diaphragm
Measuring connection for reference chamber
Reference pressure adjustment valve
Pump connection
Controller seat
Control chamber
Measuring connection for process pressure
Process chamber connection

Fig 3.28 Control characteristics of a diaphragm controller.

P₁ = process pressure, P₂ = pressure in pump, Pref = reference pressure

To achieve higher flow rates, several diaphragm controllers can be connected in parallel. This means that the process chambers and the reference chambers are also connected in parallel. Fig. 3.29 shows such a connection of 3 MR 50 diaphragm controllers.

To control a vacuum process, it is frequently necessary to modify the pressure in individual process steps. With a diaphragm controller this can be done either manually or via electric control of the reference pressure.

Electric control of the reference pressure of a diaphragm controller is relatively easy because of the small reference volume that always remains constant. Fig. 3.31 shows such an arrangement on the left as a picture and on the right schematically, see 3.5.5 for application examples with diaphragm controllers.

To be able to change the reference pressure and thus the process pressure towards higher pressures, a gas inlet valve must additionally be installed at the process chamber. This valve is opened by means of a differential pressure switch (not shown in Fig. 3.31) when the desired higher process pressure exceeds the current process pressure by more than the pressure difference set on the differential pressure switch.

Fig 3.29 Triple connection of diaphragm controllers

Fig 3.30 Control of vacuum drying processes by regulation of the intake pressure of the vacuum pump according to the water vapor tolerance.

DC - Diaphragm controller
P - Vacuum pump
M - Measuring and switching device
PS - Pressure sensor
V1 - Pump valve
V2 - Gas inlet valve
TH - Throttle
RC - Reference chamber
PC - Process chamber
CV - Internal reference pressure control valve

Fig 3.31 Diaphragm controller with external automatic reference pressure regulation.

DC - Diaphragm controller
PS - Process pressure sensor
RS - Reference pressure sensor
V1 - Gas inlet valve
V2 - Pump valve
V3 - Gas inlet variable-leak valve
TH - Throttle
M - Measuring and switching device
PP - Process pump
RC - Reference chamber
PC - Process chamber
AP - Auxiliary pump
CV - Internal reference pressure control valve

Pressure regulation in high and ultrahigh vacuum systems

If the pressure is to be kept constant within certain limits, an equilibrium must be established between the gas admitted to the vacuum vessel and the gas simultaneously removed by the pump with the aid of valves or throttling devices. This is not very difficult in rough and medium vacuum systems because desorption of adsorbed gases from the walls is generally negligible in comparison to the quantity of gas flowing through the system. Pressure regulation can be carried out through gas inlet or pumping speed regulation. However, the use of diaphragm controllers is only possible between atmospheric pressure and about 10 mbar.

In the high and ultrahigh vacuum range, on the other hand, the gas evolution from the vessel walls has a decisive influence on the pressure. Setting of specific pressure values in the high and ultrahigh vacuum range, therefore, is only possible if the gas evolution from the walls is negligible in relation to the controlled admission of gas by means of the pressure-regulating unit. For this reason, pressure regulation in this range is usually effected as gas admission regulation with an electric PID controller. Piezoelectric or servomotor-controlled variable-leak valves are used as actuators. Only bakeable all-metal gas inlet valves should be used for pressure regulation below 10^-6 mbar.

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