Tag Archive: hose

Conduit calculations (pressure losses, velocity)

Pressure losses in the conduit

Pressure losses in the conduit

These calculations can help you to estimate the pressure losses, and flow velocity in the conduit (hose, pipe or tube) and check/correct conduit ID.

Calculations Notes:

  • The recommended flow velocity in conduits you can find at the article Recommended flow velocity.
  • The assumption: pressure losses on elbows, fittings at calculated conduit is zero.
  • Height difference between IN and OUT points needs to calculate hydrostatic pressure what will be added to pressure losses, if IN point is below OUT (use positive value), or subtracted from pressure losses, if IN point is upper than OUT (use negative value). Use value “0” if height difference can be neglected.
  • To find Darcy friction factor there are different formulas used:

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Hose Overall Assembly Length

There are three different ways to measure hose assembly length are used in the practice:

  1. Use value of “Overall length” (Default for Parker’s suppliers)
  2. Use value of “Seat-to-seat length” (Default for RYCO’s suppliers)
  3. Use value of “Cut length” (Default for EATON suppliers)


Unfortunately, there are no any standards for the definition of  “Overall Length” value and therefore different manufactures determine different ways of measuring the value of Overall Length. This is why you need to be very careful before ordering hoses with new supplier and you have to specify which length you provide, because sometimes it can be critical.

The normal practice of “Overall Length” measuring please see below:

All US fittings (JIC, SAE, NPSM), except ORFS fittings, are measured up to the end of the nut:

US fittings

DIN, BSP and ORFS fittings are measured up to the end of the sealing head (this rule does not work with RYCO suppliers, but works with Parker and EATON suppliers):

ORFS fittings

SAE flange fittings are measured up to the face:

SAE flange fittings

All male threads are measured up to the end of the fittings:

male threads

All 90 degree fittings are measured to the head center line:

90* fittings

Other elbow fittings can be measured up to the center line of the head face (RYCO) or up to the center line of the seal face (Parker):

0..90* fittings

I recommend to use Parker’s rule for this type of fittings.

And my last recommendation: check the value of Overall Assembly length: it has not to be less than Minimum Overall ASSY Length:

If the Overall Length is not long enough it could hinder the ability of the hose assembly to function properly. Notes from RYCO:

This is particularly the case when utilising very short hose assemblies, where a shortening or shrinkage of the hose under pressure may result in hose and coupling separation. In addition, small misalignments, vibration and other displacements may induce very high stresses upon the hose/coupling juncture, as there is little capacity for the flexible nature of the hose to compensate.

Parameters “A” and “B” you can take from manufacturer’s catalog.


Flushing of the pipes and tubes

To do pipe flushing correctly we need to provide the special conditions to create turbulent flow. This is required to remove particles from the surface inside spindle tubes. For that we need:

  1.  High velocity of the flushing fluid (not be less than 2 to 3 m/sec. = 106 ft./sec.)
  2.  High temperature of the flushing fluid (a minimum temperature should be 140°F = 60°C)
  3.  Low viscosity of the flushing fluid (in the 10 to 15 cSt range at 104°F = 40°C)
  4.  The pressure of the flushing fluid should be held to a minimum 3 to 5 bar (22 to 73 psi), measured downstream from the flushing circuit, before the return line filter and sampling port
  5.  The flushing time to be 30 min.

Next good references about flushing I found in the internet and want to share:

ID, OD and Dash size


  • What is dash size?

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Dash size is the common method used to refer to the diameter of a hose or tube in 1/16″ increments.

For hoses this is a value for Inside Diameter, or I.D. For example, -6 hose would indicate a hose with inside diameter of 6/16″ – or 3/8″.

For tubes this is a value for Outside Diameter, or O.D. For example, a -10 tube would indicate a tube with an outside diameter of 10/16″ – or 5/8″.


By the way, dash sizes do not correspond to the exact hose inside dimension. Actual ID’s are smaller.

There are couple examples how to mark piping size at the schematic using dash size:



So, if you found at the drawing, for example, (-6) size, this means:

  • For Hose: Internal Diameter is approx. 3/8″
  • For Tube: Outside Diameter is 3/8″, therefore an Internal Diameter will be much smaller.

This is why sizes of tube and pipe have to be selected accurately, especially if you are going to connect them (for example, trough bulkhead fitting). Otherwise, you can get different flow rate and extra pressure drop in the line with smaller size.