Ellis,J. Pressure transients in water engineering, A guide to analysis and interpretation of behaviour

Подождите немного. Документ загружается.

412

(a) Swin

check valve

Cover

Hinge pin

Door

Flow

Air plug

Bod

Seat ring Face ring

(

)

External lever and wei

(

)

Inte

ral relief valve

(

)

External lever and axial s

rin

)

External torsional s

rin

(

)

Air chamber + lever and wei

(g)

linder and oil reservoir

Fig. 21.2. Swing check valve and modiﬁcations

Pressure transients in water engineering

so this part of the buoyant weight closure moment is modest. A large

opening angle means that the door has a relatively long way to travel

before reaching its seat, implying a greater time of closure. Seating

angle is often close to the vertical when installed in a horizontal pipeline

— the most common application — so that the lever arm r

sinðÞ

becomes very small as the door approaches its seat, rendering the

closure moment almost negligible at this stage. The valve may often

rely to some extent upon an actual reversed ﬂow to push the door

onto its seat. On closure, subsequent abrupt elimination of the reversed

velocity can produce valve ‘slam’ with noise, vibration, high transient

pressure and possible damage.

These features of the swing check give this type of valve a relatively

poor closure performance, especially in larger sizes. Modiﬁcations can

be made to this valve in an effort to improve closure performance

and Fig. 21.2b—g illustrates a range of these.

Two fundamentally different approaches can be adopted when

deciding the way in which a check valve will behave during closure.

The free-acting behaviour of the basic valve can be enhanced to yield

a faster response. In these circumstances an ‘ideal’ free-acting valve

would close just at the moment of ﬂow reversal. In practice there will

be some reﬂux V

, however small, when the door meets its seat.

Where this velocity produces an inertial head rise jaV

=gj, which is

413

= distance to centre of gravity

Seating angle = b

a = opening angle

= buoyant weight

dq/dt, d

q/dt

Centre of gravity

m = hinge friction

Fig. 21.3. Schematic of swing check valve door

Check valve characteristics

acceptable then the valve is considered satisfactory. On the other hand

if closure is protracted in relation to the rate of deceleration, a sub-

stantial reversed velocity V

can develop with a correspondingly large

inertial head rise when the valve shuts. Free-acting valve response

can be improved to give a smaller V

. Modelling of the pipeline

system can be carried out without detailed consideration of the speciﬁc

free-acting valve up until the moment of closure so that the methods

described in Chapter 20 can be employed without modiﬁcation.

The second option to improve closure conditions is to delay the

valve closure and allow the reversed ﬂow to develop. By controlling

the time of closure the reversed ﬂow can gradually be ‘throttled’ up

until the moment of closure. Thus as the reversed ﬂow is progres-

sively eliminated, the deceleration gradient dV=dt can be made as

low as necessary to avoid unacceptable transient effects on closure.

Response of this ‘damped’ valve has to be modelled as an integral

part of the pipeline network, including the characteristics of the

damping mechanism.

Using the common swing-check valve as an example both of the

above types of valve modiﬁcation are considered. Alterations shown

in Fig. 21.2b—e retain the free-acting nature of the valve while Fig.

21.2f and 21.2g illustrate valve damping mechanisms.

21.2.1 Free-acting modiﬁcations

An external lever and weight can be added with the lever at an angle to

the valve door (Fig. 21.2b). This enhances the closure moment and

produces an important closing moment even when the door is close

to its seat. The angle between the valve door and the lever can be

used to alter closure performance or indeed to make it easier for the

valve to open if required, although this is liable to have a detrimental

affect on closure performance. The size of weight and the length of

lever arm also inﬂuences performance. The greater the additional

closing moment provided by the lever and weight the shorter the

closure time but with the penalty of increased ﬂow velocity to get the

valve fully opened.

Two forms of external spring can be introduced. A linear spring

connected to the lever (Fig. 21.2d) or a torsional spring on the

projecting hinge shaft (Fig. 21.2e). When the valve is open the spring

is in tension and so additional closure moment is achieved without

signiﬁcantly increasing the moment of inertia of moving parts. Spring

stiffness will improve performance, with a stiffer spring improving

414

Pressure transients in water engineering

closure performance but with the requirement for a higher velocity to

open the valve against the increased resistance.

A rule of thumb sometimes used is to consider use of measures to

improve closure performance when velocity through the valve exceeds

2.5 m/s.

A pressure relief valve which opens to alleviate transient pressure rise

on valve closure, can be included (Fig. 21.2c). A variation is to have a

bypass around the valve and to include a slow-closing valve within the

small bypass line. The bypass valve remains open until ﬂow reverses,

acting as a pressure relief mechanism. The valve then closes slowly,

gradually decelerating ﬂow in the bypass.

The free-acting swing check valve is best operated with the valve

fully open, as a partly open door may tend to ‘ﬂutter’ causing accelerated

wear of the hinge.

To maintain pump prime a swing check valve or other pattern can be

included at the intake from a wet well. The valve is usually integral with

a strainer in these circumstances. Head loss through the strainer may

be 0:8V

=ð2gÞ.

21.2.2 Valve damping modiﬁcations

An air chamber can be attached to an external lever as shown in

Fig. 21.2f. As the valve opens, air can enter the chamber freely from

the atmosphere. When the valve starts to close, the air in the chamber

is compressed and pressure rises. A resistance force is produced which

increases as the valve door moves towards its seat. Controlled release

of air to atmosphere can be achieved using a ﬂow-regulating valve.

Flow through the check valve is allowed to reverse and later as the

valve door nears its seat this reversed ﬂow is gradually reduced to

zero, giving a quiet closure.

Figure 21.2g shows an oil-ﬁlled piston and oil reservoir. When the

valve is opening, a free ﬂow of oil from the reservoir to the piston

cylinder takes place. As the valve closes, a ﬂow-regulating valve

controls the rate at which oil returns to the reservoir. The piston move-

ment is also controlled and with it movement of the valve door. During

the controlled closure a reversed velocity is allowed to develop through

the check valve. As the valve door approaches its seat, this reversed

ﬂow is throttled at a controlled rate with modest deceleration rates

dV=dt to give quiet valve closure.

A modiﬁcation to the oil cylinder approach is to have the piston only

make contact with the lever during the ﬁnal stages of closure so that the

415

Check valve characteristics

door moves freely towards its seat until the ﬁnal stages of closure. The

piston can also be mounted internally to make contact with the door

itself.

A relatively slow-closing valve may allow high reversed velocity to

occur and the damping device may be unable to control the hydrody-

namic forces and can become unstable if not carefully chosen. A

quick-closing free-acting valve may be considered preferable in some

circumstances.

21.3 ‘Recoil’ valves

The closure performance of free-acting check valves having a rotating

door can be improved substantially if a number of changes are made

to the geometry of the valve. These improvements produced the

‘recoil’ check valve, an example of which is shown in Fig. 21.4a. This

valve remains suitable for both clean water and also sewage applica-

tions. The enhancements include:

(a) a weighted door which increases buoyant weight and closure

moment substantially

(b) a restricted ‘lift’ so that the valve door does not open to the same

degree as the simple swing check; this reduces travel distance

reduces the travel distance of the valve door but also ensures

that a good lever arm and thus closure moment remains even as

a door nears its seat.

In larger sizes the good closure performance of a single-door valve,

which deteriorates as diameter increases, can be retained by having

several doors. Figure 21.4b shows a three-door pattern. Doors of

these larger valves can also be weighted as well as having a similar

seating angle as the smaller sizes. The lift of the valve door can likewise

be restricted. It should be noted that closure performance of these

valves has been optimised for a particular installation and the same

closure performance cannot be expected if the valve is placed in a

vertical riser. However, valves have been produced which are optimised

for vertically upwards ﬂow and Fig. 21.4c depicts such a three-door

type.

These valves are suitable for application to water and sewage

systems with branch velocities typically 3—5 m/s. Recoil types should

be considered for high-lift, multi-pump applications.

416

Pressure transients in water engineering

21.4 Tilting disk valve

Also known as a ‘slanting’ disk, this free-acting valve also relies on rota-

tion of the valve door to achieve closure but with the hinge partway

along the valve body (Fig. 21.5).

This valve has a good closure performance on account of its seating

angle and restricted opening angle. Some versions such as the Glenﬁeld

type were speciﬁcally designed to have a low head drop.

The valve is available in wafer patterns such as the NAF check so

that reasonable space saving is achieved. The disk is light in weight

with low inertia and opens fully with a velocity of 0.6 m/s, thus avoiding

disk ﬂutter. Seating angle is 158 from the vertical (in a horizontal line)

417

(b) Multi-door check valve(a) Glenfield recoil check valve

Flow

Inlet body

Flow

Outlet body

Doo

Seat

Face

Stopper

Diaphragm

Air plug

Cover

Hinge pin

Outlet bodyInlet body

Door

Seat ring

Face ring

(

)

Multi-door check valve for verticall

wards flow

Fig. 21.4. ‘Recoil’ and multi-door check valves

Check valve characteristics

up to DN 450 and 108 for DN 500 or more. Auxiliary internal springs

can be ﬁtted to enhance closure performance, with a reduction in

pressure rise of 75% after spring addition being measured in one test.

The valve is best suited to clean water applications rather than raw

sewage.

Tilting disk valves are not generally suitable for vertically downward

ﬂow.

21.5 Rubber ﬂap valve

Bearing a superﬁcial resemblance to the swing check type is the

rubber ﬂap valve (Fig. 21.6). This free-acting valve is simple in design

and is suitable for both clean water and sewage applications. The

valve has a door which is encased in rubber and the entire valve

418

(a) Glenfield tiltin

disk valve

Body half

Face ring

Seat ring

Body hal

Disc

Air plugCover

Flow

Stop

Open

Shut

Opening angle

(b) Wafer-type body tiltin

disk

Valve fitted with auxiliary

torsional springs

Fig. 21.5. Tilting disk valves

Pressure transients in water engineering

body is sometimes designed to accept a rubber lining. Instead of

having a hinge and shaft, a rubber strip is included. One end of the

strip is connected to the door and the other end is clamped in place

at the top of the valve body. Like the swing check, the door rotates

to open with the rubber strip bending. If the door is light the buoyant

weight closing moment will be more modest than with a heavier

door. The example shown in Fig. 21.6a will not have a good closure

performance and its merits lie in its simplicity. If the seat angle is

about 458 from the vertical, as illustrated in Fig. 21.6b, and angle of

travel is 358 then the buoyant weight moment remains signiﬁcant

until closure, providing an improvement in closure performance. A

seat angled away from the vertical also helps to reduce overall valve

419

(

)

Flexible hin

e check valve

Normal flow direction

Closing disk

Ductile iron/nitrile coated

(b) Rubber flapper check valve

Normal flow direction

Flexible hinge

Free closure

Damped closure

Optional bottom buffer

Fig. 21.6. Rubber ﬂapper check valves

Check valve characteristics

door travel and thus closure time. Since no shaft projects through the

valve body, the options of having external lever and weight or springs

to assist closure are not available with these valves.

In some valves, such as shown in Fig. 21.6b, the option of installing a

‘bottom buffer’ is available. This is in the form of a piston which projects

into the valve near to the bottom of the seat. When the closing door

makes contact with the piston over the ﬁnal stages of closure,

the piston gradually moves back into an oil-ﬁlled cylinder, effectively

throttling any reversed ﬂow which develops and controlling decelera-

tion rate dV=dt. This modiﬁcation produces a damped closure over

the ﬁnal stages of movement and it is necessary to know characteristics

of the oil cylinder and piston for any modelling exercise involving valve

closure.

Being rubber-coated, door closure will not be accompanied by the

sharp crack which is present with metal-to-metal seating.

21.6 Split disk valve

Another pattern of free-acting valve which has become popular is the

split disk type (Fig. 21.7). It is composed of two lightweight semicircular

hinged doors. Valve closure is accomplished using a pair of torsional

springs. When installed in a horizontal line with the valve spindle

vertical, there is no buoyant weight moment to assist closure but

reliance is entirely on the springs. When installed in a vertically

upwards ﬂow line, buoyant weight moment of the doors assists closure.

Conversely, in a vertically downwards ﬂow, door weight retards closure.

Due to reliance on internal springs, the use of this type of valve is more

restricted, with installation in sewage schemes not advisable.

420

Shut Fully open

Normal flow direction

Torsional sprin

Fig. 21.7. Split disk valve with torsional springs

Pressure transients in water engineering

In terms of closure performance, the split disk valve is an improve-

ment on the basic swing check but does not match the recoil pattern.

Popularity of the valve rests on its lightweight, wafer design which

can be ﬁtted between ﬂanges, and its relatively lower cost. The space

saving achieved by using this valve can have an inﬂuence upon the

dimensions of pumping station buildings.

21.7 Butterﬂy valve used as a check valve

The butterﬂy valve can be ﬁtted with an external lever and weight plus

an oil-ﬁlled cylinder to damp valve motion (Fig. 21.8). A locking device

is included which prevents the valve closing until pumping failure has

occurred. Typically a butterﬂy valve might be used on a larger diameter

of line, say DN 1400 or larger. Use of the damping mechanism, as for all

valves thus ﬁtted, allows a substantial reversed ﬂow to occur for an

appreciable time. Reversed rotation of pumps should be investigated.

In one instance the pump was running backwards at 150% of its

design forward speed and it is important to ensure that this is

421

Door closed

Door open

90∞

Flow direction

Oil flow regulating valve

Oil reservoir

Lever and weight

Cylinder connected to lever

Fig. 21.8. Butterﬂy valve as a check valve (simpliﬁed)

Check valve characteristics