Introduction

One common support question we get is "What information is needed to accurately model a real-world piping system in PIPE-FLO?" The answer to that question is based on the amount of detail that's placed into the model: the more detailed the model the closer it represents the physical piping system. This typically results in a second question: "How much detail do I need in my piping system model?"

This article discusses the various items found in a physical piping system and the impact each item has on the accuracy of the piping system model. The majority of the information needed to create a piping system model can be found on the design documents that should be readily available. This includes the piping schematic drawing and vendor supplied equipment performance data, such as a pump curve.

The following elements affect the operation of a piping system:

• Piping System Connections
• Pump Performance Curve
• Component Losses
• Control Valve
• Fluid Properties
• Pipe Properties
• Valves & Fittings 

Some of the items can have a major impact on system accuracy and some have a minor impact. Our goal is to develop the most accurate model, even if we are limited to entering a minimal amount of data due to time constraints or a lack of sufficient data.

From Conception to Design to Construction to Operation

There is a lot of uncertainty at the beginning of a project and the amount of information available is much less than what is available once a system is built. During the conception phase, uncertainties exist with the size and length of piping in the system, the number and type of valves and fittings, and specific equipment data such as pump performance curves or control valve data. PIPE-FLO can be used to help size and select equipment, and as equipment is selected the model can be updated with manufacturer's performance data.

As the design firms up and the piping layout is established, the model can be updated with more accurate information. Pumps may be purchased and the curve data entered into the model. As the system is constructed, more uncertainty is eliminated and the model can be further updated.

After commissioning, actual operating data can be used to adjust and validate the model and establish a baseline for future comparison.

Design Documents

There are a variety of design documents that are very helpful in developing a piping system model.They include the process flow diagrams, Piping and Instrumentation Diagrams (P&ID), general arrangement drawings, piping isometric drawings, equipment Specification Documents, Purchase Orders, and vendor supplied operating and maintenance manuals. If these documents aren't available locally, the majority of equipment suppliers have documents such as pump curves, valve characteristics, and operating and maintenance manuals on their website. With a good search engine one can easily obtain information about much of the equipment in a plant.

Accuracy of Design Drawings

Often times a customer mentions their systems have undergone major modifications and the design drawings are not up to date. The lack of up-to-date design drawings not only makes it difficult to develop a piping system model, but it makes it very difficult to operate and maintain the system and properly train the operating staff.

With the lack of accurate design documents, it will be necessary to walk down the system and create an up-to-date flow diagram and identify the major items in this article. Once time permits you may want to update the piping system drawings to increase the accuracy. Updating design documents is an excellent task for engineering inters, it lets you update the drawings at a reasonable cost, and provides the intern with real life experience in an operating plant.

Availability of 3D-CAD Model

Many of the new piping systems are being designed with 3D-CAD software. The CAD software is used for the placement of equipment, pipe routing, locating hangers and restraints, etc. The 3D-CAD model contains the pipe lengths, number of bends, valves, fittings, and elevations. You can use the information from the 3D-CAD model to help enter data in the PIPE-FLO model of your piping system.

You will still need to enter fluid property data, performance data for pumps, components, and controls to complete your model, but the availability of the piping details offers a head start in entering the necessary pipeline data.

Major Items

The items listed in this group have the greatest effect on the accuracy of the piping system model. Each item is discussed along with the document where this information can be found.

Establish System Connection

The system connections in the piping system model are the most critical element of the model. If the various items in the model are not connected the same way as the physical piping system then there is no way the model can accurately represent the physical system. The connection information is readily available on an accurate process flow diagram, P&ID, or by walking the system. These drawings are schematic in nature and they are commonly used by everyone involved in the project to provide a clear picture of the total system.

In PIPE-FLO the connection information is entered as the piping system drawing is created within the program. Making the piping system drawing look like the P&ID or flow drawing makes it easier for everyone involved in the project to be more familiar with the model.

Each item in the physical piping system is typically designated using a unique equipment identifier. By using the same identifiers in the piping system model as used on the physical system, it is much easier to confirm that the model accurately represents the physical connections in the system and everyone involved can gain a clearer picture of the piping system model.

Pump Performance

It is extremely important to accurately represent the operation of centrifugal pumps in the piping system model. Pumps are the primary element that adds energy to the piping system in the form of head.The head developed by a centrifugal pump is determined by the flow rate through the pump as indicated on the manufacturer's supplied pump curve.

A pump curve is typically included in the purchase documents supplied by the pump manufacturer. If the pump curve is missing, the majority of pump manufactures have published their curves in PDF files available on their website. The pump curve can be printed and the performance data entered into the PIPE-FLO model.In addition, over 95 pump manufacturers have supplied electronic pump catalogs that can be downloaded from the Engineered Software website (http://www.eng-software.com/pml/default.aspx). Using this approach you simply download and open the appropriate catalog, select the pump type, speed, and model, specify the impeller diameter, and insert it into the piping system model.

Centrifugal pumps are available in a range of impeller diameters as well as rotational speeds. When entering the pump performance data from the manufacturer it is extremely important to accurately enter the rotational speed and impeller diameter, along with the head, flow and efficiency values. By entering the manufacturers curve into the piping system model, PIPE-FLO is able to calculate the total developed head for a given flow rate through the pump.

The actual pump performance can deviate from the manufacturer's pump curve as the pump ages. Cavitation, cavitation damage to the impeller, and wearing ring damage can cause the pump performance to drop off of the pump curve. This will cause a deviation of the model's results from the actual plant measurements, but this demonstrates the usefulness of PIPE-FLO as a troubleshooting tool.

Components

Components are items used in the system to accomplish the process requirements.Components can include items such as heat exchangers, filters, and strainers. These components typically are characterized by head loss (or pressure drop) as a function of the flow rate through the component, typically a second order relationship.

The purchase documents for each component in the system may have a head loss vs. flow rate graph provided by the equipment manufacturer. This data can be entered into the piping system model by entering a series of flow rate / head loss data sets in the component dialog box. Often the manufacturer will only supply a value of head loss (or pressure drop) at a single flow rate. In this case, a second order curve can be estimated for the hydraulic performance of the component. In PIPE-FLO the "Generate Second Order Curve" calculator can be used to generate the data points for a range of flows rates.

Tanks

Tanks and vessels store fluid in a piping system. Each tank is located within the plant at a specific elevation, and each tank has a range of liquid levels and surface pressures based on system operations.The liquid level and pressure establish the hydraulic energy at the bottom of the tank, and along with the tank elevation, contribute to the fluid energy at any given point in the system. The elevation of the bottom of the tank can be determined from a general arrangement drawing showing the grade level for the tank.

The pressure and level of fluid in the tank is dictated by the operating conditions of the plant. The operating level in the tank can be measured using installed instrumentation and should be accurately entered when modeling a piping system. For water, every foot of difference between the value entered into the PIPE-FLO model and the actual level or elevation in the real-world system introduces an inaccuracy of approximately 0.4 psi at any point of calculated pressure.

Where a pipe connects to the tank or vessel affects the pressure that is felt at that pipe penetration based on the level of liquid above the connection. It is important to enter the correct pipe penetration height on the tank dialog box in the configuration tab.

Pipe Diameter

The proper inside pipe diameter is very critical in accurately modeling a fluid piping system. The head loss in a pipeline is a function of the pipe length, fluid velocity in the pipeline, friction factor, and pipe diameter, as described by the Darcy Equations:


Where:

h_L =head loss in ft of fluid

f= Darcy friction factor

L= pipe length in ft

Q = flow rate in gpm

d = inside pipe diameter in inches

Since pipe diameter is raised to the 5^th power in the denominator, a slight error in the pipe inside diameter can have a major impact on the calculated head loss for the pipeline.

The important thing to remember about dealing with pipe is that it is specified by pipe material, nominal size, and schedule or wall thickness. For example, steel pipe manufactured according to the ANSI B36.10 standard is available in specific nominal sizes, and each nominal size has a set of corresponding schedules. For example 6 inch steel pipe is available in schedules 40, 80, 120, and 160, where 8 inch steel pipe is available in schedules 20, 30, 40, 60, 80, 100, 120, 140, and 160. Each pipe schedule has a specific inside diameter for the nominal size and schedule.

It is possible for various pipe materials to share the same nominal sizes, but not the same schedules. Stainless steel pipe nominal sizes are the same as the ANSI B36.10 standard for carbon steel, but the same schedules are not available. For example 4 inch nominal size steel pipe is available in schedules 40, 80, 120, and 160, where 4 inch stainless steel pipe is available in schedules 5S, 10S, 40S, and 80S.

In addition, pipe made of different material may have different available nominal sizes and schedules. For example, the available nominal sizes and wall thickness of cast iron pipe are very different than carbon steel pipe.

The pipe material used also affects the absolute roughness of the internal surface of the pipe, which has a smaller impact on the calculated head loss of the pipeline. For example, clean commercial steel pipe has a higher roughness value than the same sized pipe made of PVC. This impact on the calculated result is minor, but choosing the right material affects the accuracy of the model. Also, the pipe roughness is affected by the age of the pipe and the amount of corrosion that has occurred over the lifetime of the piping system. Again, this impact is minor but does affect the model's accuracy.

The lifetime of the piping system influences the model in another way, which may be a major impact on the accuracy. Anything that reduces the flow passage increases the resistance to flow, head loss, and pressure drop in the real world system. Sedimentation or corrosion products that reduce the effective pipe diameter, fouling of a heat exchanger, or particulate build-up in a filter or strainer will result in a pressure drop will not be accounted for in the model. Fortunately, a discrepancy between calculated results and the actual operating system is one way that PIPE-FLO can be used to troubleshoot a system.

When building a piping system model make sure the pipe material table, nominal size, and schedule are properly identified for each pipeline in the system. Much of this information can be obtained from the design drawings, P&ID, and the pipe specifications used in the project.

Block Valves & Check Valves

Block and check valves are included in the major items that affect the accuracy of the piping system model.A block valve is a valve used to isolate equipment in an operating plant. A block valve is typically fully open (to allow flow) or fully closed (to block flow).A check valve allows flow in only one direction, and closes when the direction of flow reverses. Check valves are most often found in the discharge of pumps to prevent reverse flow through non-operating pumps.

The valve design is the major factor affecting the head loss across a valve. Valves such as ball valves, gate valves, plug valves, butterfly valves, and tilting check valves have a straight flow through design which minimizes the resistance to flow and have a small contribution to the overall head loss in a system. Globe valves, diaphragm valves, swing checks, lift checks and stop checks have a more torturous flow path through the valve resulting in a greater amount of head loss. Block and check valves appear on the flow diagram and P&ID and the type of valve can be determined by visual inspection or from purchasing documents. It is important to use the correct type of valve in the PIPE-FLO model to increase the accuracy of the results.

Minor Items

The following items typically have less impact on the accuracy of the piping system model when taken individually, but discrepancies between the model and actual piping system can add up.Once again these items are arranged from most important to lease important.

Fluid Properties

The properties of the fluid passing through a piping system typically have a minor effect on the operation of a fluid piping system, but depending on the fluid and the range of operating temperatures, the effect can have a greater significance on the calculated results. The fluid properties of viscosity, vapor pressure, and density are discussed regarding their impact on the accuracy of a piping system model.

Process fluids with a high viscosity can have a major impact of the head loss in a piping system. Fluids with long molecular structures can have wide variations in viscosity with changes in the fluid temperature. The fluid viscosity impacts the calculation of the Reynolds Number and the Darcy friction factor which directly affects the head loss in a pipeline. By lowering the fluid temperature, the fluid viscosity increases and the head loss in the pipeline increases.

The fluid viscosity also affects a pump's performance. When pumping fluids with high viscosity the flow rate, pump head, and pump efficiency tend to decrease. This cause a reduction in the head developed by the pump at a given flow rate and increases the power draw of the motor.

Fluid density typically has a minor effect on the pipeline head loss. The fluid density does affect the power required by a pump, but it does not affect the pump performance curve.

The fluid vapor pressure is a function of the fluid and the temperature of a fluid. A fluid with a high vapor pressure causes the Net Positive Suction Head available (NPSHa) to decrease at the pump suction.A pump will cavitate if the NPSHa at the suction is less than the NPSH required by the pump. A cavitating pump will not operate on the pump curve and results in reducing the head and flow rate of the pump.

Fittings

Fittings are used to redirect flow or connect pipelines, tanks, and other components. Many fittings may be drawn on the flow diagram or the P&ID, but others may not be shown.For example, the entry or exit fittings where a pipeline is connected to a tank is typically not shown, but they contribute to the head loss of the system. The type of tank fitting may be found in tank specification documents or design drawings, but may have to be established by visual inspection.

Reducers, enlargers, and tees may or may not be shown on the flow diagram and should be factored into the piping system model.

The major unknown factor for fittings in a piping system model is typically the number of elbows needed to route the pipeline to the desired location. The number of elbows can be determined from the general arrangement drawings or the piping isometric drawings, if available, or by visual inspection.

Pipe Length

The pipe length can be taken off the general arrangement drawing or the piping isometric drawing which can then be entered into the piping system model.In looking at the head loss formula previously presented, pipeline head loss is directly proportional to the length of the pipeline. For example, 400 gpm of 60°F water flowing through a 100 ft length of 4 inch steel schedule 40 pipeline results in a head loss of 8.5 ft. A 10% error on pipe length would affect that value by 0.85 ft of fluid.

Control Valves

The last item in the system is the control valve. There are three things that can affect the accuracy of the calculated results for a control valve in the piping system model:

• Inserting the wrong type of control. The PIPE-FLO program is able to model the operation of Flow Control Valves, Pressure Reducing Valves, and Back Pressure Valves. If the wrong type of control is placed in the system model, then the results will not be accurate. The results of inserting the wrong type of control in a system are immediately obvious when reviewing the calculated results.
• Entering the wrong set point. If an incorrect set point is entered into the control, the results will not reflect the operation of the physical system. Since a set point on a control valve is typically displayed on plant instrumentation, entering that number into the piping system model is straightforward.
• Entering improper valve characteristics. A control valve's characteristic trim determines the flow coefficient profile over the range of operation of the valve. This C_Vprofile is supplied by the manufacturer and can be entered into the PIPE-FLO model. If this information is not correctly entered, the valve position of the model will not accurately represent the operation of the physical piping system.

Conclusion

Following the above recommendations for building a PIPE-FLO piping system model, many of our customers are able to accurately simulate the operation of their piping system. Once you know you have an accurate model, it can be used as on operation tool, training tool, as well as a maintenance troubleshooting tool. If the model was accurate 6 months ago, and it does not accurately represent the operation of the physical piping system today, either someone changed the model or the physical system has undergone changes. These changes can be traced back to incorrect valve placement, worn equipment, or instrumentation that is no longer in calibration.