frequently asked questions

FAQ

Frequently Asked Questions

 

1. General Questions and Replies with regards to Control Valves

2. Specific Questions with regards to Mankenberg Valves

 

What does Kv value mean?

The Kv value is a reference value for the flow rate of valves:

Definition

 

What does Kvs value mean?
For engineering a necessary KV value is calculated on basis of the given operating conditions. At Mankenberg and in general an allowance of 30 % is added. The Kv value of the valve should reach this Kvs value.

What is an inlet pressure pv (p1)?
The inlet pressure indicates the pressure in the system upstream of the valve.

What is an outlet pressure ph (p2)?
The outlet pressure indicates the pressure in the system downstream of the valve.

What is the differential pressure, Δp?
The differential pressure defines the pressure difference between inlet pressure and outlet pressure. If pressure ranges are given, the smallest pressure difference is relevant with regards to valve design..

What is a pressure drop?
The pressure drop indicates the active pressure difference between the effective inlet and outlet pressure.

Why is the flow velocity so important?
In pipelines certain flow velocities are admissible depending on the medium. In addition to the KV(s) value the flow velocity is a decisive value for valve dimensioning.

What is the pressure setting range?
The pressure setting range indicates the range within which a valve can be set or used.
Example:
The indication pressure range 4-6 bar means

  • for a pressure reducing valve the outlet pressure can be set between 4 and 6 bar
  • for an overflow valve (back pressure regulator) - the inlet pressure can be set between 4 and 6 bar
  • for a bleeding and venting valve the valve can be used for this system pressure range


What is the reduction ratio?
The reduction ratio indicates the max. ratio of inlet and outlet pressure which can be reduced within one step by a pressure reducing valve.
Example: Reduction ratio 20:1 (the regulation data sheet of the valve indicates 20)

Example: The nominal value for the outlet pressure 1.2 bar is the highest admissible pv = 20 x 1.2 bar = 24 bar

What is the valve seat?
The valve seat is the apron for a defined flow through opening in the valve through which the medium to be regulated has to flow.

What is the valve cone?
The valve cone is a movable element within the valve that bears on the valve seat. By means of the cone stroke the valve is opened, throttled or closed.

What is a released valve?
On a released valve the forces acting on the cone are compensated on an equally sized release surface owing to the inlet and outlet pressure. Thus the valve features considerably better regulating properties.

What does hard facing mean?
It is recommended that the cone for pressure regulating valves for liquids and with a pressure drop of abt. > 25 bar be designed with a hard facing layer. This will prevent early wear and tear due to cavitation.

What is a control line?
The control line takes the inlet and outlet pressure to be regulated from the pipeline and transfers it to the control unit.Not every pressure regulating valve requires a control line. The control line can also be used to set pressures at locations remote from the valve.

What does control unit mean?
The pressure to be regulated is transferred to the pressure regulating valve mechanism via the control unit. The pressure acting on the control surface closes (pressure reducing valve) or opens (backpressure regulator) the valve against the spring force.
Three construction types are distinguished:

  • diaphragm
  • piston with O-ring and other seals
  • bellow

    Why is it advisable to extend the pipeline in some cases?
    In many cases the valve is sufficiently dimensioned for a certain application with regards to the Kv(s) value, but it might be too small in respect of nominal diameter and the resulting flow velocities. Too high flow velocities result in noise, pressure rise and wear and tear. On a small scale the flow velocity can be reduced to an admissible value thanks to the extended pipeline. The pipeline extension is to be carried out by the customer.

    Why can a leakage line be necessary?
    In case of toxic or hazardous media the valve must be provided with a closed spring cap (with adjusting screw seal) with leakage line connection. During the on-site assembly a leakage line has to be laid which drains the medium safely in case the control unit becomes defective.

    What is the difference between a start-up and continuous bleeding valve?
    The start-up bleeding valves bleed plants with low internal pressure during start-up or filling. The float acts directly onto the cone. Such valves have a large seat diameter to ensure fast bleeding at a pressure below 0.1 bar. During operation they are kept closed by the internal pressure of the vessel. In case of a suddenly occurring vacuum the valves open and compensate the pressure. Thus damages caused by a vacuum will be avoided. The continuous bleeding valves are used to evacuate the air accumulating during operation. They are provided with a lever transmission thus they also operate at very low or very high pressures. If air intake is to be avoided the outlet is to be provided with a non-return valve. In such a case they will act as bleeding valves without venting function.

    What is Nm³ (cubic metre standard), Sm³ (American cubic metre standard) and Bm³ (cubic metre during operation)?
    Nm³: Volume of a medium (liquid or gas) at the standard state 1 bar (abs) 20°U
    Sm³: -> engl: the American variant of Nm³
    Bm³: Volume of the medium (liquid or gas) in the operating mode. i.e. at operating pressure and operating temperature.
    Attention: Bm³ and Nm³ may often differ from each other owing to the compressibility of the medium. In such a case the differentiation between Bm³ and Nm³ is of particular importance.

    What is the difference between safety valve and backpressure regulator?
    The safety valve is to safely discharge the medium at a preset nominal value during a certain pressure relief process. The overflow valve (backpressure regulator) limits the pressure upstream of the valve to the set (but alterable) nominal value. The backpressure regulator only discharges the quantity of the medium needed to attain the nominal value.

    Why is a control line needed?
    Some valve designs require control lines to safely transmit the required set pressure to the control unit. Control lines can enhance the regulating properties and are used to realise special applications (e.g. regulation of differential pressure, of quantities, or vacuums).

  • What is the difference between a pressure reducing valve and a pressure maintaining valve?
    Both valves keep the selected set pressure constant. The pressure reducing valve does so on the downstream side whilst the pressure maintaining valve does so on the upstream side of the valve (UV=backpressure regulator).

    Which parameters are important for the selection of the nominal diameter apart from the Kv(s) values)?
    The medium, flow velocity, viscosity and the pressure range to be regulated.

    What is the control deviation and how much must it be?
    In the range from 10 percent to 70 percent of the possible flow rate the pressure variation and the set value (psoll) must not vary more than + 5 percent. The control deviation indicates the extent of the variation.

    2. Specific Questions with regards to Mankenberg Valves

    Does Mankenberg also supply valves in accordance with NACE (Guidelines on Acid Gas)?
    Yes, for example made of Hastelloy C4.

    Are Mankenberg valves available made of sea water resistant materials?

    Yes, for example made of titanium, 2 54 SMO = Avesta steel 1.4547, Duplex 1.4462

    Can a pressure regulating valve be used as shut-off device?
    Yes, it can but it should not be used as such because of resulting leakages.

    Are ozone resistant Mankenberg valves available?
    Yes

    Are silicon-free Mankenberg valves available?
    Yes

    Does Mankenberg supply valves in accordance with ANSI / ASME standards?
    The valves have not been designed in accordance with ANSI/ASME but as per AD 2000. The flanges were drilled according to ANSI, and the materials as per ASTM standard.

    How do I install pressure regulating valves?
    As a basic principle pressure regulating valves are to be installed in a horizontal line, however, a vertical line is also possible for gases. If the valve is installed horizontally into a vertical line, this may result in regulating inaccuracies and increased wear and tear.
    - gases: Installation is possible with the spring cap pointing upwards or downwards or in vertical lines
    - liquids: Installation with the spring cap pointing downwards. Thus gas cushions are avoided that may cause the valve to oscillate.
    - steam: Installation with the spring cap pointing downwards to protect the diaphragm from overheating by means of a condensate guard.

    What does the notation "DM 505 1/2*250ST, 15E-0,5EV" stand for?
    Valve type, connection size, rated pressure level at the inlet side, version, Kvs value, elastomer code, control range, made cone tight, material.

    Why are our float condensate drains suitable for biogas applications?
    The valves feature a soft seal, the condensate guard has a sealing effect. The valve is gas tight, made of stainless steel and has no ignition sources.

    Our pressure reducing valves are suitable up to which temperature?
    Depending on the type Mankenberg pressure reducing valves can be used up to
    550 °C.

    Can a pressure regulating valve be set up to the max. admissible inlet/outlet pressure?
    No. Are the pressure regulating valves preset when supplied?
    No.

    Are the pressure regulating valves preset when supplied?
    No.

    If the valve is preset for a certain outlet pressure, can an alternating inlet pressure influence the regulating result?
    Yes, within the scope of the control deviation.

    Why has the control line to be connected at least 10 times the nominal diameter downstream of the valve?
    In order to minimise the influence of turbulences when the medium flows out of the valve.

    Why can I not set a pressure reducing valve from 0.5 through 10 bar like with the valves of the competitors?
    Because we do not accept the adversarial regulating properties involved.

    Is the pipeline diameter sufficient to determine the suitable bleeding/venting valve?
    Yes, if you meet the requirements of the work sheet no. 334 of the DVGW (German Technical and Scientific Association for Gas and Water).

    Why should I choose a self-energised valve of all valves?

    Such valve works self-sufficiently. No additional energy supply has to be installed (no power supply lines, no compressed air lines etc.). This saves money and minimises error sources.

     

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