Hydraulic System Design Criteria for Plants – Technical Specs

This article is about What is Hydraulic Design Criteria and technical specification used in plants and refineries.

Hydraulic System Design Criteria

What Hydraulic System Is?

A hydraulic system is a type of fluid power system that uses pressurized liquid, typically oil, to transmit power and perform mechanical tasks. It operates based on Pascal’s principle, which states that when pressure is applied to a confined fluid, the pressure is transmitted equally in all directions.

In a hydraulic system, a pump pressurizes the hydraulic fluid, creating a flow of pressurized oil through pipes and hoses. This pressurized oil is then directed to hydraulic actuators, such as cylinders or motors, which convert the hydraulic energy into mechanical energy. The actuators then perform various tasks, such as lifting heavy objects, moving machinery, or controlling the motion of equipment.

Hydraulic systems are widely used in various applications, including heavy machinery, industrial equipment, automotive brakes, aircraft flight control systems, and more. They are favored for their ability to provide high force and precise control, making them suitable for applications that require smooth and accurate movements.

Overall, hydraulic systems offer reliable and efficient power transmission, making them essential in many industries for a wide range of tasks and applications.

Hydraulic System Design Criteria

1. Line Sizes Limitations:

Pipe sizes of 1 ¼”, 2 ½” and odd pipes sizes such as 5”, 7”, 9”, 11” and 13” will not be used unless
specifically required by the process Licensor and approved by Company in writing.

2. General Line Sizing Criteria

1. Selection of pipe sizes should meet both the pressure drop and velocity criteria of hydraulic system. The total pressure drop in each system must be checked to ensure that the system meets the overall pressure drop balance, whether or not the individual process lines meet the pressure drop and velocity criteria given here.

2. The pressure drop criteria presented in the guidelines below are generally based on economic line size selection versus operating cost (Power). When a line size choice is available, or the next line size smaller is just outside the pressure drop criteria range, consideration may be given to selecting the smaller line size, provided always that the velocity criteria are met and the pressure drop evaluation in section 4.6.3~7 is compiled with.

This approach is particularly appropriate for alloy or other special material lines where the piping cost per unit length is higher. In each case where the line size proposed is outside of the pressure drop criteria, this shall be submitted to the Company for consideration and approval on a case-by case basis.

3. A safety margin of 20% will be added to all friction loss calculations. This margin relates to frictional losses only and not to head losses due to elevation or velocity change. Geodetic height shall be calculated considering lowest point and highest point under all possible scenarios.

4. Vortexing velocity: the simplest solution to a vortex is to install a vortex breaker in the draw off connection and to limit the line to a maximum outlet velocity of 1.83 m/s.

5. In general, for atmospheric and higher pressure columns, 1% of the operating absolute pressure will be used as a maximum drop per 30 m of overhead piping for hydraulic system. Overhead lines in vacuum service are generally sized on an available pressure drop, or least annual cost basis.

6. Pulsation dampeners will be provided on all reciprocating compressor suction and discharge piping. Allow a dampener pressure drop equal to: 1.67(R-1)/R, where R is the compression ratio. This may require reduction for high pressure and multistage machines.

7. Suction and discharge lines for positive displacement pumps will be sized for 160% of the pumping rate. Suction and discharge lines of proportioning pumps will be sized for 300% of the pumping rate. Pulsation dampeners should be provided on at least the discharge piping and in some cases the suction piping. Allow 1% of the discharge operating pressure for the dampener pressure drop; this may require reduction for high pressure and/or multistage pumps, as required by the process. Allowance for acceleration head will be included in NPSHA calculation.

8. Start-up, shutdown, fill, off-test and unit by-pass lines may be required in operating units. Start-up and shutdown lines will generally be sized for 50% of normal flow. Fill lines will be sized to fill system in a reasonable time, depending upon service. Likewise, with pressurizing lines, an off test header will be sized for 50% of the flow of all products into the header. Unit bypass piping will generally be sized for full flow. Lines in these services will be sized to consume the entire available pressure drop.

3. Guidelines for Liquid Flow

The following guidelines may be used to size process and utility piping for liquid service. The guidelines cover most normal situations. The total pressure drop in each system must be checked to ensure that the system meets the design pressure drop balance, whether or not individual process lines meet the pressure drop and velocity criteria given here.

Table 2 – Guidelines For Sizing Liquid Lines

Notes:

1. Pump suction line diameters should normally not be more than two standard line sizes larger than the pump suction nozzle. Pump suction lines are sized primarily by NPSH consideration; the above values are typical operating ranges. For reciprocating pumps, maximum instantaneous flow will be used.

2. Minimum velocity based on the assumption that seawater is dosed with chlorine. Higher velocities shall be subjected to vendor approval.

3. Maximum velocity shall be less than erosional velocity which is defined by the formula 100/(ρ)1/2 , ρ is the density in kg/m3.

4. Guidelines for Liquid Gravity Flow Lines

1. Effective gravity flow through piping systems depends on the recognition of two regimes:
self venting flow and siphon flow.

2. For vertical piping, self-venting criteria of hydraulic system shall be expressed as a function of velocity and gravity or reduced to pipe diameter as a function of flow (the Froude Number (Fr) will be the main design criteria). Rearranging the equation for (Fr), which must always be smaller than 0.30, gives the following criterion:

d > 0.9465 (Q/3.79)
0.4

d, pipe diameter in inches
Q, flow rate in liters per minute (l/min)

3. Short, horizontal pipelines will also become self-venting at this transition. However, the transition is very sensitive to pipe slope and somewhat sensitive to pipe length. Generally, slopes should be at least 3% with a maximum velocity of 0.3 m/s.

5. Guidelines for Vapor Flow

The following guidelines may be used to size vapor lines. These guidelines cover most normal situations, but may not be applicable to all cases. For critical services and long headers, the total pressure drop in the line must be checked to make sure that the circuit meets the design pressure balance, regardless of the pressure drop and velocity criteria given here. For long vapor lines, such as flare headers or vacuum transfer lines, when ∆ P>20% P1, a compressible flow calculation procedure such as Fanno line Calculation shall be used.

Table 3 – Guidelines For Sizing Vapour Lines

6. Guideline for Steam Lines

Following are guidelines for steam line sizing. Typical velocity ranges are presented for the more
common line sizes.

7. Guidelines for Sizing Highly Viscous and Slurry Lines.

The following table gives normal and maximum velocities for highly viscous and slurry flows. When
sizing slurry lines, be mindful that at very low velocities, the solid portion of the flow may settle out.
The minimum velocity recommended for avoiding this usually undesirable effect.

Table 5 – Guidelines For Sizing Highly Viscous And Slurry Lines

2. If erosion is of a particular concern, limit maximum velocity to 2.7 m/sec.

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