Aspen Touch Solutions ATM-123R Series Specifikace stáhnout pdf (Strana 366)

A-30 Extensibility FAQ

A-30

To illustrate this, assume that the temperature of the inlet

stream to a unit operation is changed from 10°C to 41°C.

The following sequence occurs:

a. The stream will “forget” its temperature, as well as any

variables it calculated based on temperature (e.g., vapour

fraction, density).

b. The unit operation will get an Execute pass, with

IsForgetting = True. If the unit operation asks its inlet

stream for any of the forgotten variables it will receive

<empty> (which can be an E_FAIL error, or the value -

32767).

c. The unit operation is unable to calculate any of the

variables it had calculated based on the inlet stream’s

forgotten variables. HYSYS causes these variables to be

forgotten as well. Their owners (e.g., the unit operations’s

outlet stream) will then receive a similar Execute.

d. When the Forget pass has propagated through the

flowsheet, the inlet stream recalculates using the new

temperature value. An Execute pass - with IsForgetting =

False - will propagate through the flowsheet.

19. What should my Execute do during a Forget?

Ans: Given the above description of the purpose of the

Forget pass, it would seem to make sense for a unit

operation’s Execute to return immediately if IsForgetting =

True. All of the variables it had calculated would be

‘forgotten’ by HYSYS, but recalculated during the

subsequent Execute. If a unit operation’s Execute performs

no calculations during the Forget pass, this is exactly what

will happen. Indeed, this is a valid method for a unit

operation. Neglecting to perform any calculations during a

forget will not cause any errors, but can cause some

inefficiency because more variables than necessary are

forgotten.

If efficiency becomes a concern, a solution may be to

calculate some variables during the Forget. Non-extension

unit operations generally calculate as much as possible

during their Forget, so as to minimize unnecessary

propagation of the Forget. As an example, consider a Cooler

operation that has the following specification:

•Inlet Pressure

• Inlet Temperature

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