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.
Surface equipment on the rig – more detail here http://en.wikipedia.org/wiki/List_of_components_of_oil_drilling_rigs
Because of a number of several combinations of surface equipment, four popular combinations are selected and the surface equipment coefficients for calculating pressure loss are shown in the below table.
How to determine the pressure drop thought the surface equipment
- Fine out the surface coefficient show in the table above.
- Use the following equation to figure out the pressure loss.
Where:
P = pressure loss, psi
W = mud density, ppg
Vf = viscosity correction factor
PV = plastic viscosity, centipoises
Q = flow rate, gpm
Di = inside diameter of pipe, inch
Cse = general coefficient for surface equipment
For more understanding, please following the example to calculate pressure loss through surface equipment
Surface coefficient is in case#2 therefore Cse is 8.
Mud weight = 9.5 ppg
Plastic Viscosity = 12 cp
Flow rate = 600 GPM
P = 155 psi.
Conclusion: With the given example, the pressure drop through surface equipment is 155 psi.
Reference: 
Drilling Hydraulic Books