elevated pressure pulsation with a methanol injection ... · methanol injection reciprocating pump...
TRANSCRIPT
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WITH A METHANOL INJECTION RECIPROCATING PUMP SYSTEM
ELEVATED PRESSURE PULSATION
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Authors
Walter Kelm Engineer
Kelm Engineering, LLC Friendswood, TX, USA
Ray Kelm Chief Engineer
Kelm Engineering, LLC Friendswood, TX, USA
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Methanol Injection Reciprocating Pump Pulsation Problem
• Offshore Floating Production Storage and Offloading (FPSO) high pressure methanol system
• Four quintuplex recip pumps in parallel • During commissioning, failure of Pressure Safety
Valve (PSV) components were observed • Likely issue was pressure pulsation
Discharge pressure of 323 barg
Relief valve (PSV) set pressure of 339 barg
Fixed Pump Speed: 446 RPM Flow Rate: 80 GPM per pump
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Piping System Layout
Recip Pumps
Spillback pressure control valves
Continued piping to umbilical
PSV installed on the blind end of the pump
2 liter gas charged dampener
An acoustic study was done to review the pressure pulsation at the PSV body.
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Acoustic Study Results Pressure pulsation due to pump A in operation
Pump A on the left.
Highest pressure pulsation in the pump manifold and at the PSV. Maximum of 73.2 bar peak-peak which is +/- 36.6 bar.
5-30 bar pk-pk pulsation in the piping, which is +/- 2.5-15 bar
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Review of Acoustic Study Results
290
300
310
320
330
340
350
360
0 90 180 270 360
Pres
sure
, bar
g
Crank Rotation, degrees
Instantaneous Pressure at PSV
Pressure at PSV, barg PSV set pressure, barg API 674 Design Margin
API 674 3rd Ed C.1.7 specifies a 5% design margin between the maximum instantaneous pressure, including pulsation, and the PSV set pressure
Exceeds PSV set pressure by 16 bar
PSV set 5% above line pressure with 16 bar margin
Exceeds API 674 design margin by 31 bar
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Acoustic Study Results
Baseline pulsation
Amplification due to acoustic response of piping
Pressure Pulsation at PSV Body
Pre
ssur
e P
ulsa
tion
Am
plitu
de
(bar
pea
k)
Pulsation over 16 bar peak opens PSV
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Acoustic Study Results
Baseline pulsation
Pressure Pulsation at PSV Body
Pre
ssur
e P
ulsa
tion
Am
plitu
de
(bar
pea
k)
• Acoustic resonance results in amplified pulsation.
• Resonance occurs when a pump excitation frequency is near an acoustic natural frequency.
• There is broad amplification centered near 130 Hz due to manifold ¼ wave acoustic response.
• There are specific peaks at piping natural frequencies between 110-140 Hz.
Specific peaks at piping natural frequencies
Broad amplification near 130 Hz
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Manifold ¼ Wave Acoustic Response
High pulsation at PSV (closed end)
Low pulsation at dampener (open end)
Acoustic length of pipe is 7.9 feet between areas with high and low pulsation
A pump manifold ¼ wave acoustic response is common for all pumps with a closed end on one side (PSV, blind flange, …) and an open end on the opposite side (gas or liquid dampener).
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• ¼ wave acoustic natural frequency is a classic acoustic response with a closed end on one side and an open end on the other side.
• Wavelength, λ = 4 * La La is acoustic length λ = 31.6 ft La is 7.9 ft • Frequency, f = c / λ c is speed of sound f = 130 Hz c is 4108 ft/s • This equation works fairly well when there is a
clearly defined closed and open end. • But for complex piping, acoustic modeling
must be done to accurately calculate acoustic natural frequencies.
Manifold ¼ Wave Acoustic Response
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Manifold ¼ Wave Acoustic Response
• For this application, the manifold ¼ wave frequency exists at 130 Hz due to the length of pipe from the PSV to the gas dampener.
• Due to the high pump speed (446 RPM), this frequency is near the 3rd and 4th pump harmonic(n=3,4)
fexcitation = RPM x 5 / 60 * n f3rd = 111.5 Hz f4th = 148.7 Hz
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Possible Solutions • Improve margin between line pressure and
PSV set pressure – Increase PSV set pressure – Decrease line pressure
• Reduce pressure pulsation – Add an orifice near pump discharge – Add a gas charged dampener near PSV – Add liquid flow through style dampener to
reduce high frequency pulsation
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Solutions: Improve PSV Margin • Increase PSV set pressure
– This was not possible because the PSV set pressure was selected due to the pipe pressure rating.
• Decrease line pressure – Lowering the line pressure by 36 bar would
satisfy the API 674 3rd Ed C.1.7 recommended margin
– However, the process requires a certain pressure
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Solutions: Reduce Pressure Pulsation
• Add an orifice plate downstream of dampener – Pressure drop provides acoustic damping that
reduces amplification due to acoustic resonance • Add a gas charged dampener near the PSV
– Changes the acoustic response near the PSV • Add a liquid volume
– Liquid volume reduces high frequency pulsation.
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Implemented Solution/Results Two changes were made to reduce the pressure pulsation and improve the margin
• A small gas charged dampener was installed on the PSV sense line to reduce the pressure pulsation measured by the PSV.
• The pump discharge set pressure was reduced to 302 barg.
These modifications have resulted in improved PSV life.
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Lessons Learned • For a reciprocating pump, a PSV must be set
with a sufficient margin above line pressure. • API 674 recommends a 5% margin including
pressure pulsation. • Due to the manifold ¼ wave response,
pressure pulsation on the back side of a pump can be much higher than in the piping system.
• If a PSV is used on the back side of a pump, be sure to consider pulsation when choosing the PSV set pressure.