Know-How Vacuum Control Valves

Task

Vacuum control valves regulate the negative pressure in pipes, vessels, and similar systems.

Field of application

Based on pressure reducing valves (DM): maintaining a vacuum

Based on back pressure regulators (UV): limiting a vacuum

Function

Pressure reducing valve as a basis

The vacuum to be maintained acts on the control diaphragm via the sense line connection in the spring bonnet and is in balance with the spring force. The valve is open when there is no pressure and closes as soon as the vacuum falls below the set value. The valve opens when the differential pressure to the atmosphere decreases.
Example:
Supply pressure: 8 bar
Vacuum to be maintained: -0.7 bar (0.3 bara)
Medium: Nitrogen
The control valve is open at -0.6 bar (0.4 bara) and closed at -0.8 bar (0.2 bara)

Back pressure regulator as a basis

The vacuum to be limited acts on the control diaphragm via the sense line connection in the spring bonnet and is in balance with the spring force. The valve is closed when there is no pressure and opens as soon as the set vacuum is exceeded. The valve closes when the differential pressure to the atmosphere decreases.
Example:
Supply pressure: 8 bar
Vacuum to be maintained: -0.4 bar (0.6 bara)
Medium: Nitrogen
The control valve is open at -0.5 bar (0.5 bara) and closed at -0.3 bar (0.7 bara)

Design data required for specification

• Medium
• Temperature (°C)
• Flow rate (Nm³/h)
• Inlet Pressure (bar)
• Vacuum to be regulated as the difference to atmospheric pressure (bar)
• Opens / closes to reduce the vacuum

Selection of valve type and nominal diameter

Calculate the operating performance parameter, the K_v value, with the highest flow rate and lowest differential pressure Δp (see the brochure “Calculating the K_v value”). Select a valve whose K_vs value is at least 30 % greater than the calculated K_v value. Highly viscous liquids or liquids that evaporate during decompression require extra additions. Pressure reducing valves should not be oversized. They work best in the range of 10 to 70 % of their K_vs value.

Important

Precise design taking into account the exact volume flows (caution with gases) and pressures is essential | Clarify function with the specialist from Mankenberg

Selection of nominal pressure and material

The nominal pressure stage must be higher than the maximum system pressure – without safety margins. Take into account the influence of temperature (see DIN 2401-1/DIN EN 1333).

The right material is selected with consideration of the operating conditions and the properties of the medium in the customer's plant. Plant operators must check the media compatibility of their plant components; special materials such as duplex, super duplex, Hastelloy®, or titanium may need to be selected.

For corrosion resistance see Corrosion

Selection of the setting range

For good control performance, select the setting range so that the vacuum (negative pressure) to be controlled is at its upper limit. For example, if the outlet pressure to be controlled is -0.4 barg, select the setting range 0.1 – 0.5  bar and not 0.3 – 0.9 bar.

Selection of elastomers

Select the elastomers according to the operating temperature and requirements of the medium. Gases, for example, can diffuse into the elastomers under high pressure and then cause damage when the pressure is released.

Flow velocity

We recommend the following flow velocities depending on pressure loss and permissible noise level:

• up to 10 mbar 2 m/s
• up to 100 mbar 4 m/s
• up to 1 bar 10 m/s
• up to 10 bar 20 m/s
• up to 100 bar 40 m/s

Sense line

Plan for a sense line. Connect it at a distance of at least 10 x the nominal diameter on the control side to be regulated.
To reduce the response speed of the valve, a throttle can be installed in the sense line, which must never be completely closed during operation.

The sense line should be rigid. Flexible hoses can cause vibrations.

Protection of your system

Install a safety valve to ensure that the maximum permissible operating pressure of the valve (normally 1.5 x max. set pressure) is not exceeded. The response pressure of the safety valve should be approx. 40 % above the max. vacuum of the vacuum control valve to prevent blow-off in the event of minor pressure fluctuations.
Example: with setting range 0.1 – 0.5 bar the response pressure is 1.4 x 0.5 = -0.7 barg

Protection of the vacuum control valve

To protect the vacuum control valve from damage caused by solid particles in the medium, a strainer or filter should be installed and regularly maintained.

Seat tightness

These valves are no shut-off devices that ensure a tight valve closure. In the closed position, they may have a leakage rate in accordance with leakage classes II–V as specified in DIN EN 60534-4 and/or ANSI FCI 70-2:

Leakage class II (metal-sealed double seat cone) = 0.5 % of the K_vs value
Leakage class III (metal-sealed cone) = 0.1 % of the K_vs value
Leakage class IV (PTFE-sealed cone ) = 0.01 % of the K_vs value
Leakage class V (soft-sealed cone) = 1.8 x 10^-5 x Δp x D* [l/h]
*D=seat diameter

Increased seat tightness must be expressly specified when ordering. Tightness can be significantly improved by using special cone seals and larger control surfaces.

During operation, solid particles often cause damage and seat leakage.

Shut-off

For installation, maintenance and a tight system seal, plan for shut-off valves upstream and downstream of the vacuum control valve. When closing the shut-off valves, always close the valve upstream of the vacuum control valve first. A bypass line may be necessary for emergency operation.

Leakage line

For toxic or hazardous media, the valve must have a leakage line connection. When the valve is installed on-site, a leakage line must be laid to safely drain the escaping medium without pressure in the event of an internal leakage.

ATEX

In all areas where explosive atmospheres may be encountered, equipment and protective systems must be selected in accordance with the equipment group and EPL as specified in the ATEX Directive 2014/34/EU.

Further information can be found in our Know-How ATEX.

Installation position

For gases a vacuum control valve can normally be installed in horizontal pipes with the bonnet facing either up or down.

Commissioning

Vacuum control valves should be started up and operated as smoothly as possible. Avoid sudden actuation of upstream or downstream valves.

Maintenance

Vacuum control valves must be regularly cleaned and maintained.

Oil-, grease-, and silicone-free devices

Our valves are also available in oil-, grease-, and silicone-free versions. However, some components must be lubricated to ensure product functionality. There are special lubricants for each specific application. For example, a different lubricant is used for food and pharmaceutical applications than for oxygen applications.

Simply stating ‘oil and grease-free‘ is not sufficient for targeted implementation. Therefore, when making inquiries and placing orders, please also specify the application so that the correct replacement lubricant can be selected.

When reordering and installing replacement and wear parts, please ensure that they are free of oil, grease, and silicone.

In extreme operating conditions and in all cases of doubt, please consult our technician.

Safety instructions, operating instructions, etc. MUST be observed.