Inlet Control Valve Failure Relief Analysis

Table of Contents

1. Purpose

To establish methods for determining relief loads for various automatic control valve configurations. The results are published in Project Specification 807, “Pressure Relief Valves”.

2. Inflection Point Engineering Practice

Control valve failure as a cause for overpressure is included in API STD 521, “Pressure-relieving and Depressuring Systems”. Inflection Point Engineering practice applies the guidelines in API 521 to calculate relief loads in services where the protected equipment is at risk for overpressure. Inflection Point Engineering considers that the bypass valve may be partially open at the time of failure.

The procedure below governs the majority of cases in Inflection Point Engineering projects. The design guidelines for the process technology should also be consulted. For cases not covered in this procedure or an instruction, consult the PRV Specialist.

3. Definitions

3.1 Control Valve

A control valve is a valve with actuator whose position is set automatically by a process variable controller in order to achieve regulatory control. The normal position of the valve is closed, or partially open.

3.2 Bypass Valve

A bypass valve is a valve in a parallel path to the control valve which is intended to be closed for the normal operation. A bypass valve may be a manual valve (e.g. “E” assembly) or a control valve equipped with a handwheel.

3.3 Effective Cv

Effective Cv is the product of the control valve Cv (in the wide open position) and the bypass factor. It is used to determine the flow capacity of the inlet control valve system for relief analysis.

In this procedure “gas” refers to either a non-condensible compressible phase, a vapor (in equilibrium with liquid), or a mixture of both.

4. Inherently Safer Design Principles

Control valve failure cases have the potential to set the pressure relief valve (PRV) size in many services, especially in large capacity units. The resulting loads can also affect the sizing of the relief header, especially in Hydroprocessing units.

The following design approaches may be considered to reduce the relief load.

• Increase design pressure of protected equipment.
• Eliminate the bypass valve, if not required for a specific Inflection Point Engineering purpose or operation.
• Install a restriction orifice in the bypass line, and use the orifice’s capacity instead of the standard bypass allowance.
• Lock the bypass valve closed (Note: this is permitted by customer request only. If the request is accepted, then the bypass factor (see Section 8.) is set to 1.0.)

5. Control Valve Failure – Causes and Effects

The causes for a control valve to open wide include (but are not limited to): valve sticking open, controller or transmitter fault, loss of instrument air (for “Fail Open” valves only), and operator action.

Each inlet control valve should be analyzed as an independent failure event. Simultaneous opening of multiple inlet valves is only considered when there is a common failure mode that would drive all of them open.

The effect of inlet valve failure on the relief case heat and weight balance (HWB) depends on several factors. The primary effect is an accumulation of material inside the equipment caused by the increased flow rate through the valve, since outlet control devices would be assumed to be static during the scenario (per API guidelines on response of instrumentation not connected to the initial failure event). This accumulation is from the normal fluid phase through the valve.

A secondary effect is the introduction of additional gas. For example, the loss of level in a separator immediately upstream of a fractionator could result in blow-through of gas which does not normally enter the fractionator. If the gas is non-condensible, Inflection Point Engineering assumes that the excess gas will blanket the overhead condenser and lead to a simultaneous loss of reflux.

Extra gas could be sponged out of the gas phase by excess liquid entering through the failed control valve. This effect can be assessed with a flash calculation, but it is safe to ignore any credit for this effect because it will be small.

Note: The air failure position of most control valves, as shown on the P&ID, is FC (Fail Closed). This designation shall be ignored when evaluating potential for the valve to fail open.

6. General Procedure

1. Analyze system pressures to determine if the relief case is applicable.

2. Determine the Cv value for the wide-open control valve.

3. Apply the bypass factor to get the total effective Cv, as required.

4. Calculate the flow rate across the control valve at relief.

5. Incorporate the inlet flow into the system H&WB at relief.

7. Pressure Considerations

For Inlet Control Valve Fails Open to be a valid relief case, the maximum operating pressure of the source shall exceed the PRV set pressure for the protected equipment.

a. If the maximum source pressure is less than the set pressure, then the relief case is not valid. Do not include a note on the 807 specification.

b. If the maximum source pressure is greater than the set pressure, but less than the allowed accumulated pressure, the case is valid but will not be governing, regardless of the control valve’s capacity. Add a note to the 807 listing Inlet Control Valve Fails Open as a non-governing case.

c. If the maximum source pressure exceeds the allowed accumulation pressure, then the failure is a valid design case and shall be calculated with the Cv value documented in a note.

d. If the maximum source pressure is not specified (often true for utility services), the source pressure shall be the greater of:

1. the mechanical design pressure of the source times 0.9, and

2. the normal pressure of the source times 1.1.

8. Bypass Valve Considerations

Inflection Point Engineering does not specify the Cv of the bypass valve for E-assemblies. There are various circumstances in which the bypass may be open, and to varying positions. To account for the bypass being at least partially open, the default is to multiply the wide-open control valve Cv by a bypass factor of 1.5 to obtain the effective Cv used to rate the inlet control valve service (below).

Exceptions to the use of the 1.5 factor are:

• When there is no bypass valve: bypass factor = 1.0
• Compressor spillback valves: factor = 1.2
• When the customer requests a higher bypass allowance in the BEDQ.
• When the bypass valve is specified by Inflection Point Engineering and a bypass valve Cv can be assigned (rarely done). In this case, the effective Cv is the sum of the Cv values.

9. Rating the Control Valve Service

The flow rate passing the wide-open control valve may be calculated with tool . Use the normal operating parameters for the upstream conditions, and the accumulated pressure of the protected equipment as the downstream pressure. For a utility service, the source pressure as described above should be used as the valve inlet pressure.

Rating the service requires the wide-open Cv of the control valve(s). Control valve Cv values shall be taken from one of following sources (in decreasing order of preference):

1. Project Specification 616, “Control Valves”

2. The actual valve Cv (for revamp projects only)

3. An estimated value equal to three times the Cv value reported for the normal case in the Equipment Report section in the Inflection Point Engineering hydraulics.

NOTE: If the “Design Cv” in the hydraulics is more than three times the normal Cv, use Design Cv times 1.2.

4. Other calculation (for example, see Attachment 1 for push-pull inlet valve sizing)

When the failure case involves two or more inlet control valves in parallel from a single source, the Cv value for the rating calculation depends on the control valve configuration.

• If the valves are split-ranged, assume the transmitter or controller fails to the output signal that opens the system to the highest total Cv. If the configuration contains a separate bypass valve, the bypass factor will apply.
• For a two-valve configuration where a switch selects the operating valve, and the control valves do not have handwheels, only the Cv of the largest control valve is used. If handwheels are shown, treat the inactive valve as a bypass and assign the bypass factor.

10. Evaluating the Heat & Weight Balance at Relief

For example services, see Attachment 2.

11. 807 Specification

For valid design cases the effective Cv value shall be reported in a note on the 807 specification. The following template note is provided in tool :

“Relief rate based on estimated inlet valve [valve item number] Cv + 50% margin for inlet valve bypass, total effective Cv = xxx. Contractor shall confirm PRV size when actual control valve is selected.”

For valid but non-governing failure cases (i.e. because the source or upstream pressure is less than the accumulated pressure, use the following template note in tool TZ-807-01:

“Other relief case(s) analyzed but found non-governing is/are: [list cause(s) but not the calculated relief rate(s)]”

Replace the brackets and the included template text with the valve’s item number and the failure case description (e.g. FV-001 Fails Open).

Sometimes a control valve failure case produces a relief rate that is very small compared to the other relief cases. The case should still be reported on the 807. However, if there are more than six relief cases to report, the Inlet Control Valve Fails Open case can be acknowledged as non-governing, but the effective Cv value shall be reported, using the second note above, for example:

“Other relief case analyzed but found non-governing is: FV-205 Fails Open with total effective Cv = xxx.”.

Attachment 1 Cv Determination for Push-Pull System

The gas supply for a vessel blanketing system is not included in the Inflection Point Engineering hydraulics. To determine a size for the supply control valve on a push-pull system, use the following procedure.

1. Use the appropriate instrument spreadsheet to determine the design gas blanketing rate: Tool for most services, or tool

2. Determine the equivalent Cv corresponding to this rate with T-807-08 (trial and error). Use the properties of the blanketing gas at normal supply pressure. Use the maximum operating pressure in the vessel as the downstream pressure.

3. Multiply the above normal case Cv by three to obtain the estimated size of the control valve.

4. Use the estimated Cv from step 3 (multiplied by the bypass factor) to obtain the effective Cv.

For some small vessels, pressure regulators are used in place of pressure control valves. The same type of relief case applies. Consult the Instrument Engineer for appropriate Cv values.

Attachment 2 Control Valve Service Examples

1. Compressor Spillback or Antisurge Control

The PRV on the compressor suction drum is sized for the wide-open failure of the spillback valves. Normal practice is to assume that the system downstream of the compressor is much larger than the suction drum; therefore, the normal discharge pressure of the downstream stage is the upstream pressure for the spillback valve rating calculation.

If the spillback valve system includes a manual bypass valve, the bypass factor is 1.2. This factor allows for a continuous spillback rate equal to 20% of forward flow.

2. Platforming Unit, Low-Flow Firing Mode

The PRV downstream of the low-flow firing FCV is to protect the combined feed exchanger from the discharge pressure of the net gas compressor. During low-flow firing, the valve is wide open and the bypass is not closed. The bypass factor is 1.5, but credit can be taken for the minimum flow to the CFE required for this operating mode.

3. Separator Letdown to Flash Drum

The failure of the separator level control valve () initially causes high liquid flow to the flash drum (early stage), but after the liquid level is lost then gas and liquid will pass through the valve. The later stage of the scenario is normally controlling, but both early and late stages should be evaluated. When rating the with T-807-08 for the later stage, the liquid rate will be the rate at maximum turndown, and the properties of the gas will be the normal separator gas composition. The normal operating pressure of the separator is used as the inlet pressure.

Using a process simulator create a mixed stream consisting of the normal separator outlet stream compositions, with the rates for the respective phases from the tool TZ-807-08 calculation above. Flash this stream to the accumulated pressure of the flash drum.

a. If the gas-space volume of the flash drum, after the normal separator liquid hold-up is added to it, is at least 35% of the total volume, then the relief stream for the flash drum PRV will only be the gas from the above flash. Note: sizing the flash drum accordingly is recommended to avoid significant liquid in the relief stream.

b. If the free gas volume is less than 35%, then liquid may be entrained in the relief stream. The conservative assumption is that the flash drum is nearly liquid full, so that the relief stream is the total incoming stream from the above flash. If the liquid outlet on the flash drum remains open, credit can be taken for the normal liquid exiting at maximum turndown.

Note: Blowcase method is being documented in the Unicracking Design Manual

4. Drum Blanketing System (with push-pull system)

The relief case can be caused by failure of the pressure controller or transmitter to a low signal, which drives the supply valve open and the vent valve closed. However, if the maximum supply pressure is less than the accumulated pressure of the drum, a relief rate cannot be calculated. The case becomes a non-governing case, which shall be acknowledged in a note on the 807 specification.

5. Fractionator fed from a Surge Drum or Upstream Column

See Attachment 3 for the procedure.

Attachment 3 Inlet Control Valve Fails Open to a Fractionator

The relief case for a fractionating column PRV requires consideration of both the inlet flow, and the effects of the incoming gas on the condensing system. If the overhead system can be blanketed by the excess gas, the column will experience reflux failure, which alone creates an overpressure case. The Reflux Stops case load plus the rate of excess feed gas flow constitutes the relief rate for the control valve failure.

This procedure is intended for the failure of the feed control valve. Utility valves (e.g. stripping steam) are not covered by this procedure.

1. Define the hydraulic cases for consideration (e.g. design and maximum turndown) for each process case as appropriate.

2. Follow steps 3 – 5 for each case defined.

3. Calculate the excess gas in the feed

a. Rate the control valve at relieving conditions (use T-807-08)

(1) Set upstream pressure = normal pressure in the source vessel

(2) Set downstream pressure = accumulated pressure in the column (= 110% or 116% of design pressure as appropriate)

(3) Determine the overall total Cv

(a) Obtain the Cv value for the control valve system

(b) Apply the design rules applicable to the control valve bypass

(c) If a power recovery turbine exists in the inlet line from the high pressure source, apply the design rules for the process technology.

(4) Set liquid rate = liquid entering the valve

Note: Some technologies require that the case be analyzed without liquid present; set liquid rate entering the valve = zero.

(5) Set the gas properties equal to those expected at the control valve inlet (e.g. the gas properties in the upstream vessel)

(a) If the higher-pressure vessel is a flash drum or separator, use the properties of the gas phase

(b) If the higher-pressure vessel is also a fractionator, flash the feed entering its lowest feed nozzle to the normal operating pressure of the upstream vessel. Use the properties of the gas phase from this flash. Note: if the lowest feed flashes to less than 0.1 wt% gas, then repeat with the next higher feed. If none of the feeds produces significant gas, then use the properties of the bottom (reboiler) section vapor.

(6) Use T-807-08 to calculate the gas flow that will accompany the liquid

b. Create a simulator stream for the mixed flow leaving the control valve

(1) Define the liquid (if present) with the normal liquid composition, temperature, and rate.

(2) Define the gas with the normal gas composition and temperature, and rate equal to the result of step 3.a.(4).

c. Calculate the duty at relief conditions for all exchangers in the feed path

(1) This step can be simplified by using normal duty of an exchanger if it can be rationalized that the actual duty would be less. Otherwise, rate the exchanger for the relief case conditions.

(2) If there are exchangers upstream of the control valve, then a trial-and-error calculation may be required. The suggested iteration variable is the liquid rate leaving the source vessel. However, a shortcut calculation can be done in which the control valve is rated in the absence of the exchanger, and then the resulting stream is flashed after adding the exchanger duty at relief conditions.

d. Flash the total stream at the accumulated pressure of the column and the total enthalpy after the last exchanger.

e. Separate the gas from the liquid, and retain the gas rate and properties

f. Subtract from this gas rate the normal gas rate at the feed inlet. The result is the excess gas rate.

4. Determine if condenser operation is affected by the gas entering the feed nozzle.

a. If non-condensible gas is present in the feed, the Reflux Stops case provides the base relief rate.

(1) Total relief rate = Reflux Stops relief rate plus excess gas rate entering the column at the feed nozzle. Calculate the final relief stream properties as follows:

(2) Temperature, Z and k. Calculate the relief temperature from the mass rates Wj and temperatures Tj for the Reflux Stops load and the excess gas flow result of step 4.a.(2):

where Wj is mass rate of gas

Use the same weighting formula for Z and k.

(3) Gas mole weight (MW). Calculate the MW from the mass rates of gas and MW values for the Reflux Stops load and the excess gas flow result:

NOTE: If the receiver is continuously vented to the relief header through an open line (i.e. there is no control valve), blanketing of the condenser may not be possible. If the combined flow of normal vent gas and excess gas from step 3 creates a receiver backpressure less than the allowed accumulation for the receiver, then blanketing does not occur and the relief load is according to step b below.

b. If non-condensible gas is not present in the feed, then the overhead condenser is assumed to continue in operation.

Relief rate = the volumetric equivalent of the excess gas at the conditions in step 3. Flash the normal column overhead gas composition to its dew point at accumulated pressure to get the relief stream properties and temperature.

5. Report the results on the 807 specification. Include the standard tool note reporting the valve item number and the effective Cv value.