2  Liquid Relief Valve Sizing.        3

3  Hydrocarbon Mixture Relief Valve Sizing.        8

4  Superheated Steam Relief Valve Sizing.        14

5  Two-Phase Relief Valve Sizing – Sizing for Sub-Cooled Liquid at the PRV Inlet.        19

6  Conclusion        24

7  Appendix 1 – API Standard Nozzle Orifice Sizes.        25

8  Nomenclature.        26

1. Introduction

FluidFlow software offers a fast and effective approach to automatically sizing your relief valves and bursting disks for liquid, gas, steam and two-phase flow systems in accordance with the API & ISO Standards. When you have sized your device, you can model a vendor specific valve model in your designed system. This allows you to fully appreciate the performance of your designed system in relief conditions.

The software is provided with a database of vendor relief devices which you can expand in your own time.

This document will focus on relief valves however, the software also enables you to automatically size many other piping components. Table 1 below describes the elements which can be automatically sized using FluidFlow software.

Table 1

As you are probably aware, FluidFlow is a modular program which means you can solve your system as a liquid, gas, two-phase, slurry system providing you have activated the relevant modules. This eliminates the need to re-define your model in a separate program if you wish to model the fluid in a different phase state which dramatically speeds up the design process.  

FluidFlow has been used successfully in industry since it was first launched 1984. The software has undergone extensive development since first launched ensuring the product is up to date, includes the very latest solution technology and offers engineers a fast and effective design simulation tool.

Flite Software Ltd is an ISO9001:2008 registered company.

2. Liquid Relief Valve Sizing.

Reference: Sizing, Selection and Installation of Pressure Relieving Devices in Refineries, Part I – Sizing & Selection, API Recommended Practice 520, Seventh Edition, January 2000. Pg 44, Example 3.6.2.2.

Description: In this example, the following relief requirements are given;

• Required crude oil flow caused by blocked discharge, 1800 gpm.

• The specific gravity, G, of the crude oil is 0.9.

• Relieving temperature, T, assumed to be 40oC.

• Relief valve set at 250 psig which is the design pressure of the equipment.

• Back pressure of 50 psig.

• Permitted accumulation of 10%.

• Relieving pressure of 275 psig.

• Discharge coefficient (Kd) = 0.65.

• Capacity correction due to back pressure, Kb, of 1.0.

• Back pressure capacity correction factor, Kw, = 0.97.

• Sizing assumed for no viscosity correction. Kv = 1.0.

• No rupture disk installed upstream of relief valve. Kc therefore is equal to 1.0.

In solving this condition, the API standard uses the liquid flow equation to size the single pressure relief valve. The equation is as follows;

The API standard notes that a standard orifice size of “P” (Appendix 1) should be selected since it has an effective area of 6.38 in2.

FluidFlow helps simplify the sizing of your relief valves and to demonstrate, let’s build a simple model of the system as described above.

Step 1: Build a Model of the System.

A model of this system has been developed as outlined in Figure 2.1. In doing so, the fluid “Crudo” has been assigned the model.

Figure 2.1: FluidFlow Model.

Step 2: Define the Design Parameters for the nodes.

The data entry for the main inlet boundary (reservoir node) is shown in Figure 2.2.

Figure 2.2: Inlet Boundary (Reservoir Node) Design Conditions.

As we can see from Figure 2.2, crude oil (crudo) has been assigned to the model and the fluid pressure and temperature defined as per the API Standard.

Figure 2.3 provides an overview of the data entry for the relief valve.

Figure 2.3: Relief Valve Design Conditions.

The design flow rate, set pressure and discharge coefficient have been defined as per the API Standard.

Figure 2.4 provides an overview of the data entry for the outlet boundary.

Figure 2.4: Outlet Boundary Design Conditions.

The outlet pressure has been set to 50 psig as per the API Standard.

Figure 2.5 provides an overview of the data entry for the pipework connecting the main inlet boundary to the relief valve (pipe node -1 in Figure 2.1). Note, this pipe has been kept quite short in an attempt to minimize the effect of any pressure losses which will in turn affect the overall results for our relief device.

Figure 2.5: Piping upstream of Relief Valve.

Figure 2.6 provides an overview of the data entry for the pipework connecting the relief valve to the main outlet boundary (pipe node -2 in Figure 2.1). Note, this pipe has also been kept quite short in an attempt to minimize the effect of any pressure losses which will in turn affect the overall results for our relief device.

Figure 2.6: Piping downstream of Relief Valve.

The model data entry is complete. Before we calculate this model, it is important to highlight that we can expect a subtle difference in results owing to the following;

1. The API Standard notes the crude oil as having a specific gravity of 0.9 whereas FluidFlow has calculated this to be 0.905.
2. The API standard solves the relief valve in isolation of any pipework whereas the software takes into account the effects of the connected pipework.

Whilst keeping these points in mind, let’s now calculate the model and view our results for the relief valve.

Figure 2.7: Calculated Relief Valve Results.

Note, the API Standard arrived at a calculated orifice size of 3066 mm2 whereas FluidFlow has calculated a size of 3077.9 mm2. Furthermore, when using the API Pressure Loss Model in FluidFlow, the software automatically suggests the next closest standard size match which in this case, is orifice size P. This was also the standard size (Appendix 1) suggested by the API Standard (6.38 in2).

Note that the Calculated Size is always greater than the Calculated Size at MAWP (Maximum Allowable Working Pressure) due to pipeline entry/exit losses.

Commentary:

This is just one of many design examples which FluidFlow has been tested against. It is clear from this example that FluidFlow offers a high standard of accuracy when sizing relief valves for liquid flow systems. FluidFlow also automatically determines the correction factor to be applied for viscosity which is included in the final solution.

FluidFlow also contains a database of relief valves meaning you can model specific vendor valves and study the performance of these valves in your system.

3. Hydrocarbon Mixture Relief Valve Sizing.

Reference: Sizing, Selection and Installation of Pressure Relieving Devices in Refineries, Part I – Sizing & Selection, API Recommended Practice 520, Seventh Edition, January 2000. Pg 44, Example 3.6.2.2.

Description: In this example, the following relief requirements are given;

• Required hydrocarbon vapor flow, W, caused by an operational upset of 53,500 lb/hr.

• The hydrocarbon vapor is a mixture of butane (C4) and pentane (C5). The molecular weight of vapor, M, is 65.

• Relieving temperature, T, of 348 K.

• Relief valve set at 75 psig which is the design pressure of the equipment.

• Back pressure of 14.7 psia.

• Permitted accumulation of 10%.

• Relieving pressure of 97.2 psia (670 kPa a).

• Calculated compressibility, Z, of 0.84.

• Discharge coefficient = 0.975.

• Cp/Cv = 1.09.

• Capacity correction due to back pressure, Kb, of 1.0.

• Capacity correction for rupture disk, Kc, of 1.0.

In solving this condition, the API standard uses the critical flow equation to size the single pressure relief valve. The equation is as follows;

The API standard notes that a standard orifice size of “P” (Appendix 1) should be selected since it has an effective area of 6.38 in2.

Let’s build a simple model of the system as described above.

Step 1: Build a Model of the System.

A model of this system has been developed as outlined in Figure 3.1. In doing so, a new butane/pentane hydrocarbon mixture has been created using the fluids already available in the fluids database. As the actual mixture ratio is not described in the API Standard, a mix ratio of 50% butane and 50% pentane (weight %) has been assumed.

Figure 3.1: FluidFlow Model.

Step 2: Define the Design Parameters for the nodes.

The data entry for the main inlet boundary (reservoir node) is shown in Figure 3.2.

Figure 3.2: Inlet Boundary (Reservoir Node) Design Conditions.

As we can see from Figure 3.2, the new hydrocarbon “Test Mix” has been assigned to the model and the fluid pressure and temperature defined as per the API Standard.

Figure 3.3 below provides an overview of the data entry for the relief valve.

Figure 3.3: Relief Valve Design Conditions.

The design flow rate, set pressure and discharge coefficient have been defined as per the API Standard.

Figure 3.4 provides an overview of the data entry for the outlet boundary.

Figure 3.4: Outlet Boundary Design Conditions.

The API Standard makes reference to an outlet pressure of 14.7 psia which of course is 1 ATM. We have simply set the outlet pressure to 1 ATM.

Figure 3.5 provides an overview of the data entry for the pipework connecting the main inlet boundary to the relief valve (pipe node -1 in Figure 3.1). Note, this pipe has been kept quite short in an attempt to minimize the effect of any pressure losses which will in turn affect the overall results for our relief device.

Figure 3.5: Piping upstream of Relief Valve.

Figure 3.6 provides an overview of the data entry for the pipework connecting the relief valve to the main outlet boundary (pipe node -2 in Figure 3.1). Note, this pipe has also been kept quite short in an attempt to minimize the effect of any pressure losses which will in turn affect the overall results for our relief device.

Figure 3.6: Piping downstream of Relief Valve.

The model data entry is complete. Before we calculate this model, it is important to highlight that we can expect a subtle difference in results owing to the following;

1. We have assumed a mixture ratio of 50-50 for our hydrocarbon fluid which will result in a different molecular weight etc.
2. The API standard solves the relief valve in isolation of any pipework whereas the software takes into account the effects of the connected pipework.
3. FluidFlow does not make any simplifying assumptions but solves for real gas conditions using an equation of state.

Whilst keeping these points in mind, let’s now calculate the model and view our results for the relief valve.

Figure 3.7: Calculated Relief Valve Results.

Note, the API Standard arrived at a calculated orifice size of 3179 mm2 whereas FluidFlow has calculated a size of 3188.5 mm2. Furthermore, when using the API Pressure Loss Model in FluidFlow, the software automatically suggests the next closest standard size match which in this case, is orifice size P. This was also the standard size suggested by the API Standard (6.38 in2).

Note that the Calculated Size is always greater than the Calculated Size at MAWP (Maximum Allowable Working Pressure) due to pipeline entry/exit losses. FluidFlow also displays the flow at STP & NTP conditions, the fluid phase state, density, velocity etc at the inlet and outlet of all elements.

Commentary:

This is just one of many design examples which FluidFlow has been tested against. It is clear from this example that FluidFlow offers a high standard of accuracy when sizing relief valves for gas flow systems. The ability to model any gas from the database and quickly create new gas mixture (mole or mass %) helps engineers expedite project designs.

FluidFlow also contains a database of relief valves meaning you can model specific vendor valves and study the performance of these valves in your system.

4. Superheated Steam Relief Valve Sizing.

Description: A safety relief valve must be sized for superheated steam in a large vessel at a set pressure of 110.4 barg for a mass flow rate of 69800 kg/hr, assuming 10% overpressure. The temperature of the steam is 420oC, the discharge coefficient is 0.84 and the back-pressure capacity correction coefficient is 1.0. The design outlet pressure will be 1 ATM (Static Pressure). The RV shall be sized in accordance with ISO 4126-1.

Let’s build the model of this system in FluidFlow and review the results.

Step 1: Build a Model of the System.

A model of this system has been developed as outlined in Figure 4.1.

Figure 4.1: FluidFlow Model.

Step 2: Define the Design Parameters for the nodes.

The data entry for the main inlet boundary (reservoir node) is shown in Figure 4.2.

Figure 4.2: Inlet Boundary (Reservoir Node) Design Conditions.

As we can see from Figure 4.2, water has been assigned to the model and the fluid pressure and temperature defined as per the design case presented. Note, the physical properties for water are defined in the FluidFlow database using the IAPWS relationships. FluidFlow will use these relationships and automatically detect the fluid phase state throughout the system based on the design temperature and pressure conditions, in this case, superheated steam.

Figure 4.3 below provides an overview of the data entry for the relief valve.

Figure 4.3: Relief Valve Design Conditions.

The design flow rate, set pressure and discharge coefficient have been defined as per the design case presented. The ISO 4126-1 method has been selected in this case to determine the pressure loss across the relief device.

Figure 4.4 provides an overview of the data entry for the outlet boundary.

Figure 4.4: Outlet Boundary Design Conditions.

The problem statement makes reference to an outlet pressure of 1 ATM. This design value has therefore been assigned to this outlet boundary.

Figure 4.5 provides an overview of the data entry for the pipework connecting the main inlet boundary to the relief valve (pipe node -1 in Figure 4.1). Note, this pipe has been kept quite short in an attempt to minimize the effect of any pressure losses which will in turn affect the overall results for our relief device.

Figure 4.5: Piping upstream of Relief Valve.

Figure 4.6 provides an overview of the data entry for the pipework connecting the relief valve to the main outlet boundary (pipe node -2 in Figure 4.1). Note, this pipe has also been kept quite short in an attempt to minimize the effect of any pressure losses which will in turn affect the overall results for our relief device.

Figure 4.6: Piping downstream of Relief Valve.

The model data entry is complete. Before we calculate this model, it is important to highlight that we can expect a subtle difference in results owing to the following;

• The ISO standard solution solves the relief valve in isolation of any pipework whereas the software takes into account the effects of the connected pipework.

Whilst keeping this point in mind, let’s now calculate the model and view our results for the relief valve.

Figure 4.7: Calculated Relief Valve Results.

FluidFlow has calculated an area of 1453.5 mm2 at MAWP. Note, the software has indicated that there are “No Predefined Sizes” for standard orifice size. This is simply as, unlike the API Standard, the ISO 4126-1 methodology doesn’t include a list of standard sizes, i.e. standard orifice size D, E, F, G etc.

Commentary:

This is just one of many design examples which FluidFlow has been tested against. It is clear from this example that FluidFlow offers a high standard of accuracy when sizing relief valves for two phase liquid-gas flow systems. The ability to model any fluid from the database and automatically track fluid phase state throughout your network within a single design tool helps engineers expedite project designs.

FluidFlow also contains a database of relief valves meaning you can model specific vendor valves and study the performance of these valves in your system.

5. Two-Phase Relief Valve Sizing – Sizing for Sub-Cooled Liquid at the PRV Inlet.

Reference: Sizing, Selection and Installation of Pressure Relieving Devices in Refineries, Part I – Sizing & Selection, API Recommended Practice 520, Seventh Edition, January 2000. Pg 73, Example D.2.2.2.

Description: In this example, the following relief requirements are given;

• Required propane volumetric flow rate caused by pump blockage, 100 gal/min.

• Temperature at the PRV inlet of 60 F.

• Relief valve set at 260 psig which is the design pressure of the equipment.

• Downstream total back pressure of 10 psig.

• Permitted accumulation of 10%.

• Relieving pressure of 300.7 psia.

• Discharge coefficient = 0.65.

• Capacity correction due to back pressure, Kb, of 1.0.

• Capacity correction for rupture disk, Kc, of 1.0.

• The mass flux, G, is 7560 lb/s ft2.

In solving this condition, the API standard uses the equation for sub-cooled liquid to size the single pressure relief valve. The equation is as follows;

The API standard notes that a standard orifice size of “F” should be selected since it has an effective area of 0.307 in2.

Let’s build a simple model of this system in FluidFlow and compare results.

Step 1: Build a Model of the System.

A model of this system has been developed as outlined in Figure 5.1.

Figure 5.1: FluidFlow Model.

Step 2: Define the Design Parameters for the nodes.

The data entry for the main inlet boundary (reservoir node) is shown in Figure 5.2.

Figure 5.2: Inlet Boundary (Reservoir Node) Design Conditions.

As we can see from Figure 5.2, propane has been assigned to the model and the fluid pressure and temperature defined as per the API Standard.

Figure 5.3 below provides an overview of the data entry for the relief valve.

Figure 5.3: Relief Valve Design Conditions.

The design flow rate, set pressure and discharge coefficient have been defined as per the API Standard.

Figure 5.4 provides an overview of the data entry for the outlet boundary.

Figure 5.4: Outlet Boundary Design Conditions.

The API Standard makes reference to an outlet pressure of 10 psig. This design value has therefore been assigned to this outlet boundary.

Figure 5.5 provides an overview of the data entry for the pipework connecting the main inlet boundary to the relief valve (pipe node -1 in Figure 5.1). Note, this pipe has been kept quite short in an attempt to minimize the effect of any pressure losses which will in turn affect the overall results for our relief device.

Figure 5.5: Piping upstream of Relief Valve.

Figure 5.6 provides an overview of the data entry for the pipework connecting the relief valve to the main outlet boundary (pipe node -2 in Figure 5.1). Note, this pipe has also been kept quite short in an attempt to minimize the effect of any pressure losses which will in turn affect the overall results for our relief device.

Figure 5.6: Piping downstream of Relief Valve.

The model data entry is complete. Before we calculate this model, it is important to highlight that we can expect a subtle difference in results owing to the following;

• The API standard solves the relief valve in isolation of any pipework whereas the software takes into account the effects of the connected pipework.

Whilst keeping this point in mind, let’s now calculate the model and view our results for the relief valve.

Figure 5.7: Calculated Relief Valve Results.

Note, the API Standard arrived at a calculated orifice size of 0.208 in2 whereas FluidFlow has calculated a size of 0.2072 in2. Furthermore, when using the API Pressure Loss Model in FluidFlow, the software automatically suggests the next closest standard size match which in this case, is orifice size F. This was also the standard size suggested by the API Standard (0.307 in2).

The example hand calculation makes reference to a density of 31.92 lb/ft3 for the propane at the inlet of the relief valve. Figure 5.7 notes that FluidFlow has calculated a density at this point of 31.64 lb/ft3.

The software has also detected flashing has occurred across the PRV and applied the appropriate pressure loss correlations. The fluid phase state is clearly visible at both the inlet and outlet of the relief valve.

Commentary:

This is just one of many design examples which FluidFlow has been tested against. It is clear from this example that FluidFlow offers a high standard of accuracy when sizing relief valves for two phase liquid-gas flow systems. The ability to model any fluid from the database and automatically track fluid phase state throughout your network within a single design tool helps engineers expedite project designs.

FluidFlow also contains a database of relief valves meaning you can model specific vendor valves and study the performance of these valves in your system.

6. Conclusion

FluidFlow lets you model and size relief devices in piping systems as opposed to perform relief valve calculations in isolation of any connected pipework. This can often lead to erroneous values as we need to consider the effects of the connected pipework and plant.

FluidFlow will automatically size relief devices to API & ISO Standards, suggest the next closest size match to API Standards as described in Appendix 1, track fluid phase state, includes a database of relief devices which you can expand and allows you to model vendor specific relief valves in your system. This helps engineers develop safe and reliable systems in a fast and efficient manner.

If you have a specific design application and wish to use an intuitive user friendly program to speed up your design process, contact us at: support@fluidflowinfo.com.

Testimonial:

“FluidFlow is fast, easy to use, accurate and a reliable package. The software drastically cuts design time - these benefits apply not only to the designer but also to the peer review team. During operation of the built systems, the agreement between running plant pressure readings against design data was highly accurate. That bought my full trust in the package”.

Mat Landowski, Lead Process Engineer

7. Appendix 1 – API Standard Nozzle Orifice Sizes.

When you have set the Pressure Loss Model for your relief valve to API Standard, FluidFlow solves your system based on the API equations and suggests the nearest standard orifice size match for your system.

In addition to automatically sizing your relief valves, FluidFlow allows you to model vendor valves in your system allowing you to fully understand the system operating performance when the relief valve is activated.

8. Nomenclature.

A         =         required effective discharge area in2 (mm2).

Q         =         flow rate, U.S. gpm (liters/min).

Kd         =        rated coefficient of discharge that should be obtained from the valve

manufacturer. For preliminary sizing, an effective discharge coefficient can

be used as follows;

0.65 when a pressure relief valve us installed with or without a rupture disk in combination.

0.62 when a pressure relief valve us installed and sizing is for a relief device in accordance with 3.11.1.2 of the API Standard.

Kw        =        correction factor due to back pressure. If the back pressure is

atmospheric, use a value for Kw of 1.0. Balanced bellows valves in back

pressure service will require the correction factor determined from Figure  

31 in the API Standard. Conventional and pilot valves require no special

correction.

Kc        =        combination correction factor for installations with a rupture disk upstream

of the pressure relief valve.

Kv        =        correction factor due to fluid viscosity as determined from Figure 36 in the

API Standard. Note, FluidFlow automatically determines this value during

the solution routine and applies the appropriate correction factor.

G        =        specific gravity of the liquid at the flowing conditions.

P1        =        Upstream relieving pressure psig (kPag).

P2        =        Back pressure psig (kPag).