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Further Information

Frequently Asked Questions

Q1 How many diameters are required before and after Commissioning Valves (CVs) for accurate flow measurement?
Q2 Where should strainers be located in pipework systems?
Q3 How is pressure drop calculated through a valve?
Q4 What is the maximum pressure that I can test my system at?
Q5 What is the difference between a FODRV and a VODRV?
Q6 What are the factory pressure tests for Crane valves?
Q7 Are Crane Malleable Iron Fittings suitable for lubricating oil service?
Q8 What is the standard paint finish on Crane iron valves?
Q9 What is the difference between grey cast iron and ductile iron?
Q10 Can copper alloy valves be used in diesel service?

Q1 How many diameters are required before and after Commissioning Valves (CVs) for accurate flow measurement?

Best practice is to have 10 clear straight diameters of pipework of the same nominal diameter before (upstream) and 5 clear straight diameters after the CV.

The diameters before the CV are important for flow measurement accuracy and the diameters after the CV maximise system pressure recovery. 

When installed with 10 and 5 diameters, other pipework components, i.e. bends/ control valves etc., can be ignored and have no effect on accuracy or system pressure recovery.

Where it is not possible to install with 10 and 5 diameters, they can be decreased to 5 and 3 diameters with only slight reduction in accuracy.  However, where the upstream diameters are decreased to 5 clear straight diameters, it becomes important to consider components that are installed within 5 diameters upstream of the CV.  For example, control valves (with convoluted water passage through them) would have a greater impact on accuracy than a concentric reducer or an elbow which would have a greater impact than a slow bend.

Where the downstream diameters are decreased to 3, almost all the system pressure has been recovered so the change from 5 to 3 has little impact.

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Q2  Where should strainers be located in pipework systems?

Strainers are installed to protect items of plant and to help ensure system efficiency by keeping heat emitters (terminal units, i.e. fan coil units etc.) free from particle build-up within them.

When used to protect items of plant (boilers / chillers / pumps etc.) strainers should be installed upstream of the item to ensure that larger particles contained in the flowing water do not enter and damage the item of plant.

Strainers can also be installed at strategic places, i.e. branches or in each terminal sub-circuit, throughout the system to ensure that smaller particles contained within the flowing water do not pass into the terminal units affecting their efficiency.

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Q3 How is pressure drop calculated through a valve?

The pressure drop across a valve is related to the flow rate passing through it, i.e. the greater the flow rate the higher the pressure drop and the lower the flow rate the lower the pressure drop.

Manufacturers publish a Flow Coefficient value to allow the calculation of pressure drop based on flow rate.

For metric units, i.e. m3/hour and bar or l/s and kPa, the flow coefficient is Kv, whereas, for imperial units, i.e. gpm and PSI, it is Cv.

As l/s and kPa are most commonly used, we will concentrate on Kv and not Cv.

The formula for calculating pressure drop is: Δp = (36Q/Kv)2
Where:

1. Δp = pressure drop (kPa)
2. Q = flow rate (l/s)
3. Kv = flow coefficient (no units)

An example is:

A flow rate of 0.225l/s passes through a D921 ¾” DRV (Double regulating Valve), calculate the pressure drop when the valve is fully open (FO).

1. Look up Kv value for ¾” D921 (Fully Open)
     • Kv = 3.61
2. Δp = (36 x Q / Kv)2
3. Δp = (36 x 0.225 /3.61)2
4. Δp = 2.2442
5. Δp = 5.035 kPa

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Q4 What is the maximum pressure that I can test my system at?

Pressure testing has implications on all of the individual system components, i.e. pipes, fittings, valves, coil etc., therefore the pressure rating of all the components needs to be considered when deciding on a suitable test pressure.
There are two pressure tests relating to valves:

  1. System Pressure Test – to test integrity of the finished assembled system, i.e. ability of the completed assembly of individual components to withstand pressure.
  2. Valve shut-off Pressure Test – to test the ability of a valve, typically an Isolation Valve, to close against a pressure.

Crane manufacture valves to allow system pressure testing to be carried out at 1.5 times maximum working pressure of the valve, i.e. for PN16 valves the test pressure can be 16 x 1.5 = 24 bar. Note that all valves must be in an open position.

The shut-off pressure test can be carried out at 1.1 time maximum working pressure, i.e. for a PN16 valve the test pressure can be 16 x 1.1 = 17.6 bar.

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Q5 What is the difference between a FODRV and a VODRV?

FODRV is the abbreviated term for Fixed Orifice Double Regulating Valves and VODRV for Variable Orifice Double Regulating Valves.

The term ‘Orifice’ refers to the opening in the valve that the water flows through.

  • FODRV: With a FODRV the ‘orifice’ that creates the measured pressure drop, is an actual opening of a fixed size, hence the term ‘Fixed orifice’. Fixed Orifice Valves have a single value for the flow coefficient Kv (see Q5). For the FODRV this value is usually referred to as Kvs.
  • VODRV: With the VODRV the ‘orifice’ that creates the pressure drop is the seat / disc assembly within the valve. As the gap between the seat and disc is different for different handwheel positions, the flow coefficient Kv is changeable depending on the handwheel position, hence the term ‘Variable Orifice’.

Because the FODRV does not have a changeable Kvs value, the accuracy remains at ±5% irrespective of the handwheel position, whereas, for the VODRV, which has a variable Kv value, the accuracy is ±5% when fully open but reduces to approximately ±18% when the valve is closed to its minimum set position, i.e. 25% open (CIBSE recommendation).

Commissioning Engineers find it easier to work with FODRV because design pressure drops can be calculated before attending site, whereas, with the VODRV the pressure drop must be re-calculated during the commissioning process each time the handwheel position is changed.

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Q6 What are the factory pressure tests for Crane valves?

Valves have slightly different test requirements, but in general pressure tests are as follows:

Copper-Alloy Valves:

For valves DN65 (2.1/2”) and above with a minimum pressure rating of PN32 the valves are tested hydrostatically (water) pressure shell 1.5 x PN rating for a specified time, related to valve size that is set down in BSEN 12266. Seat tests, where applicable, are 1.1 x PN rating for a specified time as per the same standard.

Generally valves DN50 (2”) and below with pressure rating on PN25 and below can be tested as above or on air at 6 bar shell and seat, if applicable for time specified in BSEN 12266. Crane generally test at 6 bar air for production speed and volume with sample test on hydrostatic.

Iron Valves:

Iron valves are tested hydrostatically (water) pressure shell 1.5 x PN rating for a specified time, related to valve size that is set down in BSEN 12266. Seat tests, where applicable, are 1.1 x PN rating for a specified time as per the standard.

Please refer to the sales office for specific details. 

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Q7 Are Crane Malleable Iron Fittings suitable for lubricating oil service?

Malleable iron fittings are suitable for lubrication oil service. Please refer to the Crane website for pressure and temperature ratings for our range of malleable iron fittings.

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Q8 What is the standard paint finish on Crane iron valves?

The paint standard for Crane iron gate, globe, check and strainer valves is as follows:

Primer dry film thickness = 30 microns minimum. Total thickness of primer and colour top-coat is between 75 to 125 microns.

The paint standard for Crane iron butterfly valves is as follows:

Fusion-bonded epoxy (FBE) coating with a thickness of 150-250 microns.

In general, the standard paint finish on Crane iron valves is suitable for most indoor environments. Please seek advice for external applications or aggressive environments such as high humidity levels.

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Q9 What is the difference between grey cast iron and ductile iron?

The difference between grey cast iron, often referred to as cast iron, and ductile iron is the addition of trace elements. Grey iron has a random flake graphite formation. In ductile iron, the addition of a trace element, typically magnesium, results in round graphite structures rather than flakes. This results in a material with superior mechanical properties to grey cast iron and similar corrosion and wear resistance. Both cast iron to EN 1561 and ductile iron to EN 1563 are acceptable materials to use for valves. Crane valves comply with relevant design standards which specify certain allowable cast and ductile iron grades to meet those standards and the requirements of the Pressure Equipment Directive (PED). The selection of materials can also relate to manufacturing processes, valve size and pressure rating.

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Q10 Can copper alloy valves be used in diesel service?

In general, copper alloy valves can be used on diesel fuel service. However, valves containing EPDM (O rings for example) are not suitable for diesel use.

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