Applies To
Product(s):	AutoPIPE
Version(s):	ALL;
Environment:	N/A
Area:
Original Author:	Bentley Technical Support Group
Date Logged & Current Version	Feb. 2017 10.01.00.09

Problem:

When performing a fluid transient, there is an option to choose, Linear Rise or Sine Rise

Which one should be used and why?

Solution:

Fluid transients are dependent on multiple parameters to correctly calculate the surge pressure attained when flow rates suddenly change. One of those key parameters would be the Rise Time, This is the time it takes the pressure wave of the transient to reach the peak value. The next important parameter is the assumed shape of the pressure wave between initial pressure to peak pressure. The 2 types of assumed curves shapes available in AutoPIPE are [L]inear / [S]ine Rise. The difference between how the two functions calculate the pressure values during the pressure rise can be visualized as below (arbitrary values for valve closure time (1 sec) and pressure):

Linear Selected:

With linear selected, the pressure rise shape (at the valve or at a bend point) is a straight line from 0 pressure to Maximum Pressure value or simply reduce the area available for flow from 100% to 0% during the rise time / valve closing time.

Example:

Start pressure = 0
Maximum pressure MaxP = 1000
Rise time (valve closing time) = 1 sec

Draw a straight line from 0 to 1000 pressure in a time duration of 1 second. Pressure value at s specific time is calculated as MaxP*t/trise

Sine Selected

With Sine selected, the pressure rise shape (at the valve or at a bend point) is calculated as a sine function shape. At each time, the pressure value is calculated using the following equation:

MaxP * Sin((PI/2)*(t/trise))

Or, mimics the actual area/velocity of fluid instead of a linear trend

Which option should be used and why?

Again, one has to assumed a curve shape from initial pressure to peak pressure. This decision should be influenced by a component's pressure drop versus percent closed, or the shape and area of the flow path from Start Time to Rise time of the transient .

Example: Ball valve:

For more details information on this setting, see AutoPIPE help mentioned below:

Topics covered by this help section include:

Linear Rise Function, Rise Time Less Than Lag Time
Linear Rise Function, Rise Time Greater Than Lag Time
Sine Rise Function, Rise Time Less Than Lag Time
Sine Rise Function, Rise Time Greater Than Lag Time

Example:

See the following results take from AutoPIPE example model, Apham1.dat, where the only change made was the Rise setting from S to L:

Using Sine Rise, results on TIH curve and forcing function file

Using Linear Rise, results on TIH file

Conclusion:

Generally, Sine function would provide a more conservative estimate of the differential pressure (see AutoPIPE help referenced information above). Whether to select a Sine or a Linear shape function would be up to the designer may be based on answers to questions like:

1. Which of the two functions provide a closer approximation of the actual pressure wave profile?

2. What type of valve closing mechanism is in place i.e. linear, equal percentage, convex etc. and how do these mechanism relate to the pressure wave shape.

Additionally one can perform some sensitivity tests to confirm that a model may be susceptible to huge changes when using one method was used over the other.

Linear shape is the simplest type of pressure wave profile that could be employ. Sine pressure wave profile may be slightly closer to actual pressure profile.