Supplemental information
Some of the information that follows is excerpted for
your reference from the iPRSM handbook. For context-sensitive information on any of these topics, pick Help on any iPRSM page.
Automatic control failure
For each automatic control failure case, give the scenario a unique extension name to help identify which control valve has failed.
Each control valve that requires calculations should have its own scenario.
Control valves that don’t result in relieving cases may be listed together in the general automatic control failure scenario. For this scenario, clear the Apply Scenario checkbox. Be sure to include appropriate Scenario Notes to document cases that don’t apply.
When calculating flow through a control valve, use the Installed Cv
for the control valve at 100% open. Wide open Cv from the manufacturer’s data should used and may be obtained from the control valve spec sheet. This will be shown as the valve Cv, not as the Cvat maximum flow.
If the manufacturer’s data is not available, the values in Table 1, provided represent a conservative assumption for globe style valves, and may be used if the specific valve data cannot be located. Do not use these values until after you have attempted to obtain specific data on the valve. Be sure to include appropriate comments in the Scenario Notes if assumptions are used.
When using a Cv from Table 1, record in the Equipment Data Notes
that specific valve Cvdata was not available, and that,
conservatively, the Cv for an <X> inch full port globe valve is used.
Use the Cv for the port diameter if known and the pipe size if unknown.
TABLE 1: FULL PORT GLOBE VALUE CV DATA
This table lists the calculated Cv (for liquid) and Cg (for gas) values for globe valves. For other types of valves such as ball and gate, use the equations in Crane Technical Paper 410.
Full Port
d^2/sqrt(K) Assumed Fisher
Steam
PROTECTED SYSTEM CONTINGENCY SCENARIO VIEW
When entering scenario data in iPRSM:
use a unique name for the scenario to help identify which control valve has failed
Hazard Type:Control Valve Failure
Flow Type:Vapor or Liquid
Select the control valve for evaluation by picking its checkbox in the Selected Control Valves list. To be available for selection, the control valve must be linked to the protected system as ancillary equipment.
iPRSM has equations to calculate the flow rates for automatic
control failure for liquids that do not flash and vapors. The equations assume that the entire pressure drop is taken across the control valve. The required relief rate for a failed control valve is typically the flow rate through the failed open valve less the normal flow rate.
A control valve failure simultaneous with a blocked outlet is normally considered a double jeopardy.
Use W adjust in iPRSM to increase or reduce the control valve flow as needed to account for normal flows.
W adjustis normally used to subtract the normal flow rate through the control valve being considered, but can also be used to increase the flow rate for relief when needed. Value must be negative if subtraction is desired.
To be conservative, use the Low Normal flow rate for the credit.
Record in the Scenario Notes that the required relief rate is the difference, as well as the source of data for the normal flow rate used, like spec sheet for equipment or control valve, PFD, etc.
MULTIPLE CONTROL VALVES IN COMBINATION
Remember that you cannot take credit for the correct response of a control valve, so even though one of the two valves is not considered to fail, it is not double jeopardy to assume that one does not respond.
Series
Depending on their set pressures, often one of the two control valves in series is full open.
There are two possible methods for determining the maximum flow through two control valves in series.
The easiest method to use to determine the flow is to combine the Cv’s of the two valves using the following formula.
Cv combined = 1/ SQRT (1/CV1^2 + 1/CV2^2 + 1/CV3^2 ….)
The Cg is computed based on the combined Cv calculated above, and C1 = 34 for a globe valve, etc.
There is some error in this method of calculating Cg.
A more rigorous approach is to iterate the flows through the two valves until they balance. This is not normally required as a first-pass calculation.
Parallel
For two or more control valves in parallel the Kv or Cv can be calculated as:
C= Cv1 + Cv2 + …
ABNORMAL HEAT INPUT
Abnormal heat input is a special case of control failure involving the flow of fuel or heating medium to process heat transfer equipment.
The heat input is limited for these cases by either a limit in the heat transfer capacity of the equipment, a supply limitation in the fuel or heating medium, or a combination of both.
The heat transfer of the equipment may be adjusted based on the temperature difference of the hot side supply vs. the cold side fluid bubble point temperature at relief pressure.
Distillation reboiler: If the normal bottoms composition bubble point temperature exceeds the hot side supply temperature, the column feed composition should be used.
For screening calculations, assume there is an infinite supply of the heating medium so the hot side temperature is the supply
temperature, or condensing temperature at supply pressure for steam systems.
If the relieving capacity of the system is adequate based on the screening calculations, the more rigorous and time consuming calculations need not be performed.
The spreadsheet referenced for blocked outlet vaporization can be used to predict the relief flow rate.
DISCUSSION SKETCH
INSTRUMENT AIR FAILURE
For the global instrument air failure scenario, evaluate the effect of all of the control valves in a system going to their failure position.
Often this results in no relieving case. Nonetheless, evaluate each of your automatic control failure scenarios to see if any cause relief situations with the valve in the design failure position.
Also look at other overpressure sources such as compressors that might cause a relief if they were to fail.
EXAMPLE
It is likely that plant-wide instrument air failure might shut down the process gas compressor in an ethylene unit.
Instrument air failure is a global failure in which the high variable back pressure should be used.