unsteady state flow in pipelines

8/3/2019 Unsteady State Flow in Pipelines

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Unsteady State Flow In

Pipelines


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)2/(2

gV


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In a typical emergency pump shutdown scenario, thelow pressure (Down-surge) can cause seversubatmospheric pressure, and column separation can

occur beside severe high pressure (Up-surge) can occurupon vapor pressure collapse.

Protective equipment is often necessary to provide fluid

and head to the system upon the down surge and alsoto bleed water out of the system during upsurge.

Most often the best protection for this situation iseither the hydropneumatic tank or surge tank.


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Hydropneumatic tank Gas vessel Air chamber

Its a vessel contains fluid at the bottom and

entrapped gas (Usually Air or Nitrogen) overlying theliquid.


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Surge tank

Its a vessel contains fluid with free surface subjectedto the atmospheric pressure


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The energy is stored in a form of pressure head either inElevation + Water level(in surge tanks)

Elevation + Water level + Pressure head(in hydropneumatic tanks)

The stored energy in the gas pressure allows for a highhydraulic grade in a relatively small tank

Either the hydropneumatic tank or the surge tank typically willoperate at normal pipe line pressure

Hydropneumatic Tank with Bladder


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Hydropneumatic Tank with Bladder-A flexible and expandable bladder is sometimes used to keepthe gas and fluid separate in the hydropneumatic tank. Since

there is no contact between the compressed air and the water,there is no dissolution. There is thus no requirement for apermanent regulation system such as an air compressor, whichis otherwise typically required (since the gas slowly dissolvesinto the water

-When using a bladder, a 'pre-charge' pressure is first applied,before the tank is connected to the system and submitted topipeline pressure. Transient protection performance when using

a bladder type tank tends to be sensitive to the pre-chargepressure, since it determines the initial gas volume andsensitivity to pressure changes. Sometimes you may have arequirement on the pre-charge pressure, such as being 5% ofthe normal pipeline pressure


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OPERATIONThe installation of such a surge vessel is very simple, but mustbe performed with care. If well done, future checking of the

vessel will be very easy.

1- Initially the precharge pressure must be adjusted to thevalue resulting from the hydraulic analysis (prechage can be

either compressed air or nitrogen). At this stage the bladdercontains no volume at all.


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2-When the gate valve is opened the water will enter the vesselunder static conditions, and begin to compress the gas (staticpressure is always higher than precharge pressure)..


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3-The water entering the vessel will further compress the prechargedgas until a balance between the liquid and the compressed gas isreached

4 Immediately after a pump stop the pressure in the line will start


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4-Immediately after a pump stop the pressure in the line will startto decrease ant the elastic energy in the vessel will discharge thewater from the vessel into the line. This prevents dangerous lowpressure along the pipe work.

A th b l th fl ill


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5- As the pressure way become very low, the flow will reverse,this will then enter into the vessel via a reduced diameter ( drillednon return valve or bypass) if hydraulically required. Several

oscillations may occur before static state is reached.

6-When the pumping station will restart, the vessel will continue to


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p p gfill until dynamic steady state is reached and it is then once againprepared for the next pump stop.

M d li C id ti


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Modeling Considerations

- The tank is preferred to be modeled directly on the main line

without adding a short connecting pipe which may lead toexcessive adjustment in the pipe length or the wave speed, whichmay intern have an impact on the results.

-The Hydropneumatic tank is typically installed just


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The Hydropneumatic tank is typically installed justdownstream the pump station to keep the water columnmoving upon pump shutdown.- The hydropneumatic tank location is uncertain if morethan one tank is required.

Dark black line = physical elevation.Dashed black line = steady state / initial conditions head.

Differential Orifice


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-The piping connection between the hydropneumatic tank andthe system should be sized to provide adequate hydrauliccapacity when the chamber is discharging, as well as to cause a

head loss sufficient to dissipate transient energy and preventthe chamber from filling too quickly. Both of theserequirements are met through the use of a piping bypass asdepicted below.


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- The "minor loss coefficient" that you enter is used for tank outflows. Fortank inflows, the minor loss coefficient is multiplied by the "ratio of losses"and the resulting coefficient is used. The effect of a differential orifice can belarge for some systems. Consider the below profile, showing the maximum

transient head for a pipeline during an emergency pump shutdown event.The inlet orifice size was decreased by 75mm and a minor loss coefficient of1.5 was used, with a ratio of 2.5. As you can see, it helps reduce the maximumtransient pressures in the system. This could also mean a possible reductionin total required tank size

-In HAMMER, the headlosses associated with this can be modeled by usingthe "Minor Loss Coefficient", "Ratio Of Losses" and "Diameter (Tank InletOrifice)" attributes of the hydropneumatic tank. This is referred to as thedifferential orifice, because the ratio of losses allows you to have the inflow

headlosses different from the outflow headlosses. In the above illustration,you can see that the check valve causes inflows to undergo largerheadlosses as water passes through the bypass. So, the ratio of lossesattribute is usually above 1.0 and applies to inflows.


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As mentioned above, in many cases a hydropneumatic tank may be

implemented only for transient protection. During a steady state condition,

the tank may simply operate under the corresponding normal / steady

state head ("line pressure"). So, for simplification, it is sometimes

preferable to select "true" for the "treat as junction" attribute in the

tank properties. Doing this allows the initial conditions solver to compute ahydraulic grade at the tank location, and the user simply assumes that the

tank has already responded to the hydraulic grade and the air volume has

expanded or contracted accordingly. In this case, the user only needs to

enter the initial volume of air under the "transient" section of the tank

properties that corresponds to that initial conditions hydraulic grade

(unless using a bladder). It is important to remember that the tank is only

treated as a junction in the initial conditions. During the transient simulation,

it is still treated as a hydropneumatic tank. Basically treating it as a

junction in the initial conditions is another way of establishing the initialhydraulic grade. The transient simulation will use that hydraulic grade

along with the gas volume as the starting conditions. The gas will then

expand and contract accordingly during the transient simulation, based on the

gas law. If you already know the hydraulic grade that you'd like to use as the

initial conditions, you would choose "false" for "treat as junction?" and enter

it under the "physical" section of the tank properties. The initial conditions

solver will then compute the flow/head in the rest of the system, with the

hydropneumatic tank as the boundary condition. In this case, the tank will

likely have either a net inflow or outflow, to balance energy across thesystem. So, your transient simulation may not begin at a true "steady"

condition.


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Elevation

-The elevation from which to calculate pressure in the


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The elevation from which to calculate pressure in the

hydropneumatic tank (typically the bottom of the tank.) It could be

set to the estimated water surface, since the air pressure (used in

the gas law equation) is above that point. However, the bottom

elevation and water surface are typically very close, so this likely

will not make a noticable difference.

Minor losses coefficient

Usually use 1.0Tank Calculation Model

- Specifies whether to use the gas law or a constant area

approximation method during EPS initial conditions. The constant

area approximation uses a linear relationship; the user must

specify minimum/maximum HGL and the corresponding volume

between. The gas law model is non-linear and follows the gas law-

as gas is compressed, it becomes harder to compress it more.

Atmospheric Pressure Head

-When using the gas law tank calculation model, this field

represents atmospheric pressure at the location being modeled.

This is required because the gas law equation works in absolute

pressure, as opposed to guage pressure.

-Treat as Junction

- Selects whether or not the tank is treated as a junction during the

initial conditions. If "false," the "HGL (Initial)" or "Level (Initial)"

field is used for the initial head. If "true," the initial conditions

solver acts as if the tank is a junction and computes normal/'line

pressure.


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Hydropneumatic Tank with

BladderVolume (tank)

-This represents the total volume of the tank. This isonly used in an EPS simulation (to find the gas

volume so that the gas law equation can be used) or

when using the bladder option ("Has Bladder?")

during a transient simulation. When using a bladder

tank, HAMMER assumes the bladder occupies this

full tank volume at its "preset pressure," so this fulltank volume value is needed by the gas law equation.

(Pre-set) pressure

-is first applied, before the tank is connected to the

system and submitted to pipeline pressure. Transient

protection performance when using a bladder type

tank tends to be sensitive to the pre-charge pressure,since it determines the initial gas volume and

sensitivity to pressure changes. Sometimes you may

have a requirement on the pre-charge pressure, such

as being 5% of the normal pipeline pressure


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Diameter (Tank Inlet Orifice)

Thi i h i f h i b h l d h i


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- This is the size of the opening between the gas vessel and the main

pipe line. It is typically smaller than the main pipe size. It is used to

compute the correct velocity through the tank, so the correct headloss is

computed based on the minor loss coefficient. Minor Loss Coefficient

(Outflow) (Version 8i)

- This is the 'k' coefficient for computing headlosses using the standard

headloss equation, H = kV2/2g. It represents the headlosses for tank

outflow.

Ratio of Losses

-This is the ratio of inflow to outflow headloss. For flows into the tank

(inflows), the "minor loss coefficient" is multiplied by this value and the

losses are computed using that. For flows out of the tank, HAMMER

only uses the "Minor Loss coefficient". So, if you enter a minor loss

coefficient of 1.5 and a ratio of losses of 2.5, the headloss coefficientused when the tank is filling would be 1.5 X 2.5 = 3.75.

Gas Law Exponent

- refers to the exponent to be used in the gas law equation. (the 'k' in

PV^k = constant) The usual range is 1.0 to 1.4. The default is 1.2.

Volume of Gas (Initial)

When not using a bladder, the initial volume of gas is an important

attribute. This is a required input field, representing the volume of gas

inside the tank at the steady state pressure (initial conditions hydraulic

grade minus tank physical elevation). During the transient simulation,

this gas volume expands or compresses, depending on the transient

pressures in the system. For example, consider a 500 L tank with base

elevation of 20 m and initial hydraulic grade of 70 m. This means that

the air pressure head is ~50 m. So, the user needs to decide how much

space (volume) the entrapped gas pocket would take up, at this pressure.


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unsteady state flow in pipelines

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