Straight run for vertical pump suction piping?

StressGuy · Wednesday at 1:15 PM

I've had a rather strange issue come up and I'd like to see how it compare with everyone else's experience. I'm sure most of us here are aware of API 686 for "machinery installation" and the requirements for piping design. Specifically the 5D of straight run requirement.

Chapter 6, Section 3 has notes for specific piping requirements for different types of machinery. 3.1.2.6 specifically calls out the 5D requirement for pump suction piping...or so I've always been told.

I have an issue now with a top suction pump where the piping designer has laid out the piping with the elbow directly on the pump nozzle. When I challenged him, he said that the rotating equipment engineer told him that this note doesn't apply for top suction and pointed me to figure B.3 in chapter 6. I've attached a snapsht here.

His assessment is that note 9 regarding the straight run requirement is only showing on the horizontal suction version and not the vertical suction. And, looking at the figure, that is indeed the case. The other notes like 1 and 2 and 4 are explicitly called out on both the horizontal and vertical schematics, but 9 is not.

The text of section 3 is only labeled as "suction piping" and doesn't not mention horizontal/vertical, so I've always been directed that it applied to vertical suction lines as well as horizontal.

What are you guys seeing in your designs? Have I been mislead and overcomplicating top suction layouts my whole career? It would make life a lot simpler for sure.

As a further complication, this B.3 figure says it is for "Typical Machinery" indicates pumps/compressors/blowers. Notes 9 and 13 are the only ones that appear to be pump specific. Point 3.2.4.1 calls for straight run on suction of centrifugal blowers and compressors. It also makes no distinction for vertical and horizontal and that point is not referenced on the B.3 figure at all. We've always done 5D minimum (some compressor vendors have required more) for compressor suctions whether they are horizontal or vertical.

It seems weird that that straight run would be required for all compressors, but only horizontal suction pumps.

LittleInch · Wednesday at 1:41 PM

See this. https://www.eng-tips.com/threads/straight-run-at-suction-of-pumps.393848/

Looks to me to be very vendor dependant, but the internal flow path of a top suction pump usually has a 90 degree element to it so they seem to be less picky about the 5D.

So you need to see what the pump vendor recommends for his particualr pump. Or fit a flow conditioning plate.

goutam_freelance · 2025-04-04T05:51:32+0100

A straight length of at least 3D is required for vertical suction lines also. This is to prevent flow disturbances, cavitation, and reduction in efficiency.

Note 9 above mentions "Pump suction straight line requirement (3.1.2.6)"
Neither note 9 nor Cl 3.2.1.6 differentiate between vertical or horizontal suctions. It is implied that this requirement applies to both vertical and horizontal suctions.

shvet · 2025-04-04T06:16:53+0100

How is the swirl oriented relative to the impeller(s)? Was this pump tested for uneven flow?
Was the impact on NPSHR and bearing&seal service life measured or accouted for? Is there a NPSH margin to compensate for an NPSHR encrease? Is there a design margin to compensate for a penalty in pump performance? Is this pump intended to operate near the boundaries of the allowable operating region?
Do you understand that this design is a gray area? Who will be accountable for pump underperformance, impeller cavitation damage, and/or bearings/seal failure, in case?
What is your personal ultimate goal? good pump operation or something else?

Hydraulic Considerations
Suction Piping
In general, pumps should have an uninterrupted and unthrottled flow into the inlet (suction) nozzle. Flow-disturbing fittings should not be present for some minimum length. Flow disturbances (Figure 3-2) on the inlet side of the pump can lead to:
• Deterioration in performance.
• Damage to the impeller and shortened impeller life (especially with high-suction-energy pumps).
• Shortened mechanical seal life.
• Shortened bearing life.
... The most disturbing flow patterns to a pump are those that result from swirling liquid that has traversed several changes of direction in various planes. Liquid in the inlet pipe should approach the pump in a state of straight, steady flow. When fittings such as “T” fittings and elbows (especially two elbows at right angles) are located too close to the pump inlet, a spinning action or “swirl” is induced. This swirl could adversely affect pump performance by reducing efficiency, head and net positive suction head (NPSH) available. It also could generate noise, vibration and damage in high-suction-energy pumps.
Elbows must be vertical when next to a double suction pump (Figure 3-5). It is always recommended that a straight, uninterrupted section of pipe be installed between the pump and the nearest fitting. This should follow the minimum straight pipe length recommendations of the Hydraulic Institute Standards (Refs. 3-2 and 3-3), as summarized in Tables 3-1 and 3-2, and Figure 3-6. ... they range from:
• One to eight pipe diameters (for low suction energy/ low specific speeds).
• Three to 16 pipe diameters (for high suction energy/ high specific speeds).
The specific straight pipe length recommendation depends on the type of fitting(s), the pump type, the suction energy level and pump specific speed ...

... optimum pump performance also requires that proper suction/inlet piping practices be followed to ensure a steady uniform flow to the pump suction at the true required suction head. Poor suction piping can result in separation, swirl and turbulence at the pump inlet, which decreases the NPSHa to the pump and causes added cavitation. High suction energy pumps are obviously most sensitive to the effects of poor suction piping (Ref. 5-11).
Figure 5-14 shows test data on the effect of poor suction piping on a small, high suction energy, end suction pump ... Suction pressure pulsation levels for normal piping (five diameters of straight pipe) are compared to the levels for suction piping with a short radius elbow mounted directly on the pump suction flange, and two short radius elbows at right angles, also mounted direct on the suction nozzle. The greater the flow disturbance entering the pump, the higher the pressure pulsations, noise and vibration, in the region of suction recirculation.

taken from https://a.co/d/ixx3PzS

Offtop
Mech engineers are always so innocent. They tend to believe that finding&following thoroughly an instruction (doesn't matter which) issued by a body (doesn't matter which) is enough to prevent claims from being made against them.

shvet · 2025-04-04T09:45:14+0100

Let me express in another words
No a straight run => swirl at the suction nozzle => flow maldistribution or assymetric at an impeller eye => cavitation and pump underperformance => impeller/seal/bearings damage => huge money losses => somebody shall take the responsibility

Ask the piping engineer how he/she believes the swirl is related to "upward vs sideward" suction orientation.

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Straight run for vertical pump suction piping?

StressGuy

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