Pressure Drop through Surface Equipment

Pressure drop through surface equipment is one of the components in drilling hydraulics that must be considered. When we talk about surface equipment, we usually refer to those following equipment as a stand pipe, surface hoses, a swivel, a gooseneck and a Kelly.

119 Pressure drop throug surface equipment 1

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Understand Pressure Loss (Frictional Pressure) in Drilling System

Frictional pressure is pressure loss acting in the opposite direction of fluid flow and today we will look into each component in pressure therefore you will get clearer picture about the frictional pressure in drilling system.

Let take a look at the simple diagram below.

Understand Frictional Pressure in Drilling 1

A mud pump creates power to move drilling fluid from point A to C and the frictional pressure or pressure loss is the amount of pressure required to transfer fluid.

Pump pressure is 2,000 psi at the starting point (“A”) and at the end point (“C”), pressure is 0 psi. This tells you that you need 2,000 psi to overcome the frictional pressure in order to move the fluid from point “A” to point “C”.

“Differential pressure between two points in the system is pressure loss while fluid is moved from one to another point.”

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Pressure Drop Across a Bit

Pressure across a bit occurs when the drilling mud passing through the jet nozzles. This pressure drop is important for drilling hydraulic optimization (maximum hydraulic horse power or impact force) and hole cleaning. The pressure drop at the nozzle area is a function of total flow area of the bit, flow rate and mud density.

Pressure drop across the bit can be determined by the following equation:

Oilfield Unit

Pb = (Q²×W) ÷ (12031 × A²)



Pb = pressure drop across a bit, psi

Q = flow rate, gpm

W = mud weight, ppg

A = total flow area, square inch

Please use the following information to determine pressure drop across a bit

Flow rate = 800 gpm

Mud weight = 9.0 ppg

Total Flow Area = 0.3728 square inch

Pb = (800²×9.0) ÷ (12031 × 0.3728²)

Pb = 3,446 psi

Metric Unit

Pb = (Q²×W) ÷ (7.191 × A²)


Pb = pressure drop across a bit, KPa

Q = flow rate, lpm

W = mud weight, kg/m³

A = total flow area, mm²

Please use the following information to determine pressure drop across a bit

Flow rate = 3,000 l/m

Mud weight = 1,100 kg/m³

Total Flow Area = 240 mm²

Pb = (3,000²×1,100) ÷ (7.191 × 240²)

Pb = 23,901  KPa

The new version of spreadsheet can be downloaded by clicking the image below. 

Ref books: 

Lapeyrouse, N.J., 2002. Formulas and calculations for drilling, production and workover, Boston: Gulf Professional publishing.

Bourgoyne, A.J.T., Chenevert , M.E. & Millheim, K.K., 1986. SPE Textbook Series, Volume 2: Applied Drilling Engineering, Society of Petroleum Engineers.

Mitchell, R.F., Miska, S. & Aadny, B.S., 2011. Fundamentals of drilling engineering, Richardson, TX: Society of Petroleum Engineers.

Type of Flow in Drilling Hydraulics

Flow of drilling fluids can be classified into one of three phases which are laminar, transition and turbulent flow.

Laminar Flow

Laminar flow, sometimes known as streamline flow, occurs when a fluid flows in parallel layers, with no disruption between the layers. At low velocities the fluid tends to flow without lateral mixing, and adjacent layers slide past one another like playing cards. There are no cross currents perpendicular to the direction of flow, nor eddies or swirls of fluids. In laminar flow the motion of the particles of fluid is very orderly with all particles moving in straight lines parallel to the pipe walls.

Transition Flow

Transitional flow exhibits characteristics of both laminar and turbulent flow. The edges of the fluid flow in a laminar state, while the center of the flow remains turbulent. Like turbulent flows, transitional flows are difficult, if not impossible, to accurately measure.

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Flow Regime and Critical Reynolds Number for Drilling Hydraulics

Drilling fluid has three flow regimes which are laminar, transition or turbulent flow. The following illustrations demonstrate three types of flow

In 1883 when Mr Osborne Reynolds did the fluid study, he discovered Reynolds number describing flow of water in a circular pipe. From his experiment, the turbulent flow starts at the Reynolds Number of 2,000 and the complete turbulent flow occurs at the Reynolds Number of 4,000. Therefore, when the calculated Reynolds number of fluid is between 2,000 and 4,000, the transition flow is called.

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