Summary

Fittings such as elbows, tees, valves and reducers represent a significant component of the pressure loss in most pipe systems. This article discusses the differences between several popular methods for determining the pressure loss through fittings. The methods discussed for fittings are: the equivalent length method, the K method (velocity head method or resistance coefficient method), the two-K method and the three-K method. In this article we also discuss method for calculating pressure loss through pipe size changes as well as control valves.

Introduction

Fittings in pipelines such as valves, elbows and changes in pipe size all cause pressure losses to fluid flowing through them. This article describes 4 methods by which the pressure losses caused by general fittings may be estimated. The equivalent length method, the K-method, the 2K-method and the 3K-method. Generally speaking the methods increase in accuracy in the order in which they are listed above. With the 3K method providing the most accurate results, particularly for large pipeline sizes. The flow coefficient method is also discussed briefly in this article.

The Flow Coefficient method, Cv and Kv, may also be used for to calculate the pressure loss through valves or other items where this information is available.

Pipe size changes cause a pressure loss which is often dependent on a number of factors. It is not easily captured in the methods used for general fittings and must be calculated separately.



Which Methods Should I use to Estimate Pressure Drop?

There are two main factors which will determine which method is appropriate for the estimation of pressure loss through fittings: the accuracy required and the resources available.

General Fittings, Tees, Elbows, Standard Valves

There are many methods available for calculating the pressure loss through standard fittings such as tees, elbows and valves. The method chosen will depend on a number of factors, such as calculation method and the data available.

Generally speaking, and with the widely published information, the 3K-method is the most accurate, and if a computer is available the 3K method is easily performed via a spreadsheet. If quick hand calculations are required and a high degree of accuracy is not required then the equivalent length or excess head (K-method) may be more suitable.

Where detailed data is available for the fittings being considered such as tables of data for the equivalent length of fittings at a number of flow rates and fittings sizes the results may be more accurate than using the 3K method, as the data is more specific.

For very large or complicated calculations, such as dynamic process modelling or large pipe network models it may be desirable to use a simpler calculation method, such as the excess head method, to reduce the running time of the model.

A description of each method and a table of typical fitting values are provided in these articles:

Equivalent Length Excess Head (K-Method) 2K Method 3K Method

Expanders, Reducers and Other Size Changes

The pressure losses through pipe size changes depend on a number of factors including the upstream and downstream pipe size, the geometry of the size change, friction factor and Reynolds Number, depending on the case. Because of these factors the pressure loss through pipe size changes is not calculated using the generic methods used for standard fittings.

Formulae are available for calculating the pressure loss through Expanders, Reducers and Other Size Changes.

Control Valves and Speciality Items

The Flow Coefficient method, Cv and Kv, may also be used for to calculate the pressure loss through valves or other items. This method is generally used when the data for that specific item is available, for example, when a control valve vendor provides the Cv value for a specific model.

Calculating Pressure Loss using Cv and Kv (Flow Coefficient)

Converting between Cv, Kv and Excess Head (K-value).



Further Reading

  1. Piping Calculations Manual (McGraw-Hill Calculations)
  2. Piping Systems Manual
  3. Flow of Fluids Through Valves, Fittings, and Pipe (TP-410 (US edition))