Spirax Sarco - Catalogo Agosto 2017

General information

Useful calculation data

��

11.5.5

ISO 9001

Cert. No. LRQ 0963008

= Mass flow in kg / h

= Upstream pressure in bar a

= Downstream pressure in bar a

= Valve flow coefficient.

- P

TI-GCM-08

CM Issue 1

The chart below is for displaying Examples 1, 2 and 3 only, a

complete sizing chart is shown overleaf.

For Water Sizing Chart see TI-GCM-09

Saturated Steam

Sizing Chart

Let:

��

�

��

� ��

��



i.e. 10 bar g = 11 bar a.

The chart overleaf shows that with a given upstream pressure P

and

with a pressure drop across the valve more than is needed to give

critical flow conditions, or  > 0.42, the steam flowrate is directly

proportional to the Kv of the valve. Conversely, with a given Kv, the

flowrate is directly proportional to the upstream pressure P

So for critical flow, we have:-

= C x Kv P

and in the units shown, C = 12 (Constant).

Thus:

= 12 Kv P

With a smaller pressure drop, the flow is reduced until it becomes

zero, at zero pressure drop. Many formulas are in current use to

predict the relationship between flowrate and the pressure drop ratio

 under these conditions. One empirical formula which gives results

very close indeed to the British Standard method, but simplifies the

calculation, is:-

= Pressure drop ratio =

Note: To convert gauge pressure to absolute pressure, add 1,

= 12 K P

√ 1 - 5.67 (0.42 - )2

If this formula is used when P

is below the value which gives critical

flow, then the term within the bracket (0.42 - ) becomes less than

zero. It is then taken as zero, and the function within the square root

sign becomes 1.

How to use the chart

Example 1:

How to find Kv value for a critical flow application.

Steam demand of heat exchanger

Steam pressure upstream of valve

Steam pressure required in exchanger

Using the selection chart opposite:

Draw a horizontal line from 800 kg/h

Draw a horizontal line from 9 bar a to the critical pressure drop

line, which is reached before a pressure drop line for (9 - 4 = 5 bar)

and drop a vertical line from the intersection to meet the 800 kg / h

horizontal.

Read the Kv at this crossing point, i.e. Kv = 7.5

Refer to the Kv values given on the appropriate Technical Information

Sheet for each valve type.

SA (self-acting), EL (electric/electronic) and PN (pneumatic) valves

may be selected using their maximum Kv values.

Example 2:

How to find the Kv value for a non-critical flow application.

= 800 kg/h

= 8 bar g = 9 bar a

= 3 bar g = 4 bar a

Steam demand of heat exchanger

Steam pressure upstream of valve

Steam pressure required in exchanger

Using the selection chart opposite:

Draw horizontal lines from 230 kg/h, and from 6 bar a to pressure

drop of (6 - 5 = 1 bar). Drop a vertical line from the intersection to

meet the 230 kg/h horizontal, and read the Kv at this crossing point,

i.e. Kv = 4.

Example 3:

How to find the pressure drop across a valve with a known Kv.

= 230 kg/h.

= 5 bar g = 6 bar a

= 4 bar g = 5 bar a

Steam demand of heat exchanger

Steam pressure upstream of valve

Kv of valve to be used

Using the selection chart opposite:

Draw horizontal lines from 11 bar a, and from 3 000 kg/h to meet

Kv = 40 line. Draw a vertical line upwards from the intersection to

meet the 11 bar a horizontal. Read the pressure drop at this crossing

point, i.e. ∆P = 1 bar (approximately).

= 3 000 kg/h

= 10 bar g = 11 bar a

= 40

Local regulations may restrict the use of this product to below the conditions quoted.

In the interests of development and improvement of the product, we reserve the right to change the specification.

��

�

��