Dynamic Modelling of Control Valves

Andy RowlandTechnical Lead – Water Networks

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Pres

sure

(m)

Element Flow (l/s)

Observed PRV d/s Pressure Against Flow

Background

• Focus is often on model calibration under normal flow conditions. BUT Models are frequently used to identify and resolve system capacity issues.

• Use and influence of K / Cv values – consequences of improper use

• Fixed outlet PRVs that are performing sub-ideally must be responding to external influences and can’t be altogether random!

Growth / Resilience / New Developments / Fire Flow / Incident Management

Why typically, is low priority given to this value that essentially quantifies valve capacity

Could the valves also be responding to flow and / or upstream pressure influences in addition to the ideal control setting?

INVESTIGATEMECHANISMS INFLUENCING

PRV OUTLET PRESSURE

• 2016 Conference Workshop started a detailed look at the modelling of PRV’s. Thoughts that came to mind at the time:

Dynamic Modelling

Understanding and improving network resilience / the ability for the network to absorb unplanned events.

Planning and designing for growth / high demand scenarios

Network optimisation and energy management

Accurately modelling the response of assets to external influence

Increased model confidence across broad range of demand scenarios

Delivering:

Which is important when:

Operational response modelling

Current Standard Practice

For PRV’s (but applicable to most control valves)

• Correct Diameter• Use software default k• Manufacturers wide-open k• Use achievable max-opening k

Capacity

• Fixed outlet element for PRVs without an external modulation controller

• Flow / time modulated elements to model actively modulated PRVs.

• Fixed throttle to simulate d/s variation• Variable modulation elements to handle

uncontrolled valves with d/s variation

Modulation(responsiveness)

* Level of widespread use

Representing the MODULATION function of a Control Valve

To throttle or not to throttle:

Head-loss induced across a throttled valve is a function of flow

Head-loss increases exponentially as a clear function of flow

Introducing a throttle downstream of the control valve we inadvertently model a flow-modulated downstream pressure.

Synergi VQ and InfoWorks FMV control valve elements both have the facility to define Flow Influenced outlet pressures:

AREvarying outlet pressures

ACTUALLYa function of Flow?

Representing the MODULATION function of a Control Valve

Example: 80mm Cla-Val NGE - “fixed outlet”

INLET HEAD 72M - STABLE

VOLATILE (but low) FLOW

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0 0.5 1 1.5 2 2.5

Pres

sure

(m)

Element Flow (l/s)

Observed PRV d/s Pressure Against Flow

Notice rate of pressure reduction reduces as flow

increases

Notice with throttleRate of Pressure Reduction increases with increased flow

Notice Extrapolation

Calibrated / Dynamically ResponsiveOUTLET HEAD 31m – 34m

Similar Example in InfoWorksVariable Pressure Profiles Observed

Downstream of a PRV most likely indicate PASSIVE FLOW MODULATION

Reasons for Passive Flow Modulation

Fixed Throttle

Pilot(Variable Throttle)

Diaphragm

Main Valve

Spring & Diaphragm

Vent to air (closed)

Pilot Isolation Valve

Model modulates K up and down to achieve set outlet pressureUser defined K(min) represents – maximum valve open position

Default Wide-Open(Manufacturer)

k – 18.6

Calibrated(80% Open)

K - 31

Representing CAPACITY limitationsThe importance of K: Software Default

Representing CAPACITY limitations

Verifying Calibration of K• Max recommended design

opening – 70%

• Observable deterioration starts at 70% opening (7l/s, k=50)

• Max opening achieved stabilises at 80% (k=31)

Given this PRV is performing as expected

Reasonable to assume that c. 80% maximum opening is typical this

style of valve under pilot operation

Representing CAPACITY limitations

Evidence of Upstream Pressure Profile in Downstream Profile:

• Evidence of Upstream Pressure Profile in Downstream Profile (shouldn’t happen on functioning PRV)

• PRV has no (or very limited) capacity for modulation – no evidence of control

• K is primary parameter for addressing capacity issues. Set K = 175 with a set pressure slightly above maximum observed to achieve calibration

Indicates that this 80mm PRV is stuck at 64% open

SUMMARY

Can Control Valves be Modelling Dynamically?

• Capacity is primarily a function of valve geometry and can be satisfactorily estimated in K. Using no more than 80% open value for traditional style valve would be appropriate to limit risk associated with over predicting PRV capacity.

• Software has the capacity to represent d/s head variation that can be linked to the influence of flow

• Most variations downstream of a functioning PRV can be related to the influence of flow (inconsistent with THV performance)

• Locked or seized PRVs can be modelled by selecting an appropriate K to represent the fixed valve opening position

• PRVs are generally predictable in their behaviour if not ideal

SUMMARY

Can Control Valves be Modelling Dynamically?

TO HAVE CONFIDENCE USING MODELS FOR:

Resilience Analysis Incident Modelling Developer Services Growth Analysis Fire flow modelling Network Optimisation Capital Investment Solution Development

Control Valves MUST be modelled dynamically