16 replies to this topic

[DylanYang]

I have to fill the pump operating condition form.

operating conditions:

liquid: water capacity = 101 GPM
head = 207.57
discharge pressure = 90 PSIG
suction pressure = -1.97 PSIG

Max allowable shut off head:______

But I don't what the maximum allowable shut off pressure is and how to fill it. I know the Max shut off head. Is it the same thing?

[Art Montemayor]

Max allowable shut off head can be the same as Max shut off head. It depends on what you are trying to state - or to protect.

You don't want the shut off head on a centrifugal pump (I have to assume this is a centrifugal pump, with 207.57 feet of head at the operating point - since you fail to identify this in your post) to exceed the maximum allowable pressure for your pump's discharge system. That, more than anything else is what is of concern with the maximum attainable pressure reached with a shut off of the pump's discharge.

[DylanYang]

ok, thanks for your answer !!
that's true my pump is centrifugal type and my operating head is lower than 207

so the MAX allowable shut off pressure can be tranfer to max shut of head right?

so, it's the same thing right?

[kkala]

Some probably useful notes in addition to the above.
1. At the design point flowrate= 101 GPM and head = 207.57 ft of water.
Differential pressure 90+1.97=91.97 psi = 6.4662 kgf/cm2 = 212.1 ft of water of sg=1.0. So pumped water must have s.g ~ 0.978.
2. Shut off head under above conditions is the head developed by the pump at zero flow, let us assume it as 250 m of water (sg~0.978). Exact value can be seen on the performance curve, given by pump supplier.
3. Nevertheless design point is usually considered at low low liquid level (LLLL) of suction tank. Suppose that HHLL (high high liquid level) of the tank is 33 ft above LLLL. Then max shut off pressure corresponds to 250+33 = 283 ft of water = 122.7 psig (suction tank is assumed atmospheric).
4. Max shut of pressure shall be not higher than design pressure of pump casing or of downstream piping (*). If a value of max allowable shutoff pressure is written, it must reflect one of the two, being higher than 122.7 psig.

Note: A pump per API 610 has casing design pressure much higher than 122.7 psig, to my information from office.
(*) PRV at pump discharge is not judged to be proper in this case.

Edited by kkala, 27 April 2012 - 03:18 PM.

[fallah]

Max allowable shut off head:______

DylanYang,

Shut off head (differential head) of a fixed speed pump with specified impeller diameter would be an unique value (intersection of the pump curve and head vertical axis), then applying "max" or "allowable" for it is meaningless. Obviously, discharge pressure at pump shut off could be various depend on suction pressure value and fluid density and can be limited to downstream design pressure.

Fallah

Edited by fallah, 29 April 2012 - 01:43 AM.

[kkala]

Shut off head is not depending on suction tank level, contrary to shut off pressure (points 2, 3 of post No 4). So post No 4 gives some clarifications on shut off pressure, too, probably useful.

[DylanYang]

i have more question

could you tell me, what is the maximum sunction pressure


and what is the definition?

[kkala]

Suction pressure is the pressure exerted at pump suction nozzle (center). Let us consider a suction pipe from a tank ending to one pump. Maximum suction head, equal to (HHLL elevation - suction nozzle center elevation), occurs when
α. pump is shut off (no friction in suction pipe), and
β. liquid level in the tank is at maximum (HHLL).
Evidently (max suction head * density * gravity acceleration) makes max suction pressure.

It is pointed out that suction piping design pressure is often nowadays considered same as discharge design pressure for safety reasons, especially for pumps in parallel - e.g. operating and installed spare. This concerns the design pressure only, and can be looked into http://www.cheresour...t-off-pressure/ '> http://www.cheresour...t-off-pressure/ .

[ankur2061]

Dylanyoung,

The maximum suction pressure can be defined as follows:

Max. Suction Pressure = Max. Operating pressure of the vapor in the vessel vapor space + Static Height from the Maximum Operating Level of the Vessel upto the pump suction nozzle centerline - Friction losses in the Suction Line

Notes for the terms used above

1. If the maximum operating pressure in the vapor space of the vessel cannot be ascertained and if there is a relief valve in the vapor space or vapor dome of the vessel then use the set pressure of the relief valve as the maximum pressure of the vapor in the vapor space of the vessel.

2. Max operating liquid level could be at High High Liquid Level (HHLL) signal from the Level Indicator of the vessel or upto the overflow nozzzle of the vessel whichever is higher

3. For friction losses in the suction line consider the lowest operating viscosity of the liquid at the highest operating temperature.

Hope this helps.

Regards,
Ankur.

Edited by ankur2061, 29 April 2012 - 12:41 PM.

[kkala]

Concerning post No 8, it is clarified that the tank is considered atmospheric, as in post No 4. Supposing that pump suction nozzle (center) is 3 ft below tank LLLL (while HHLL 33 ft above it), head corresponding to max suction pressure is 33+3 = 36 ft of water (sg~0.978). So max suction pressure would be 36*0.3048 m * 978 kg/m3 * 9.81 m/s2 (gauge)= 105275 Pa g = 15.3 psi g.

I think following is useful concerning post No 9 (clarifications).
1. Max operating pressure over liquid is 0 psig, in case of an atmospheric tank.
2. Elevation difference of tank HHLL has to be taken from pump suction center up (not from bottom up).
3. Max suction pressure (in pump data sheets) is considered here at shut off , which means no consideration of friction losses in suction line (no flow). This is more conservative and then helps in estimating max shut of pressure. "Normal" suction pressure is specified at rated pump flow. In case you specify max suction pressure at rated flow, can you please notify?

Edited by kkala, 29 April 2012 - 02:48 PM.

[S.AHMAD]

Dear all
1. My understanding is to consider the worst scenario.
2. The worst case scenario in this particular case is when the PSV popped due to liquid overfilling. Where the pump suction pressure will be the PSV set pressure plus allowable accumulation (e.g 10%) plus the highest liquid level e.g. at the PSV orifice.
3. However, some company gives exception if the vessel is equipped with an independent high level alarm coupled with sufficient time (from high level alarm to the highest point) for operator to take action

[ankur2061]

I think following is useful concerning post No 9 (clarifications).
1. Max operating pressure over liquid is 0 psig, in case of an atmospheric tank.
2. Elevation difference of tank HHLL has to be taken from pump suction center up (not from bottom up).
3. Max suction pressure (in pump data sheets) is considered here at shut off , which means no consideration of friction losses in suction line (no flow). This is more conservative and then helps in estimating max shut of pressure. "Normal" suction pressure is specified at rated pump flow. In case you specify max suction pressure at rated flow, can you please notify?

1. That is obvious.

2. Point no.2 is confusing. If the suction source is above the pump suction nozzle then the static height from the highest liquid level upto the pump suction nozzle centerline will be added for calculating the maximum suction pressure and if the suction source is below the pump suction nozzle then the static height from the highest liquid level upto the pump suction nozzle centerline will be subtracted for calculating the maximum suction pressure.

3. In context off shut-off head the friction loss may be neglected. However, post # 7 by OP only asks what is the definition of maximum suction pressure without specifying the context for a pump, which obligates considering the friction loss in the pipe for calculating the maximum suction pressure.

Edited by ankur2061, 29 April 2012 - 11:34 PM.

[kkala]

Agreed to the post No 11 by S.AHMAD. PSV accumulation (para 2) is often neglected here, but it is correct to be considered. Other cases of PSV opening can be high temperature, e.g. an LPG drum or a forced recirculation pump in a water tube boiler (rare boiler). Sufficient time (para 3) is understood to be 15-20 min.

Comments on post No 12 by ankur2060:
1. Suction pressure of -1.97 psig indicates atmospheric tank at suction, so example referred to such a tank.
2. "Suction source" is not clear, but you may mean suction tank. It is improbable (and not a good practice) for an industrial pump to have its suction nozzle center higher than tank HHLL. Even so, the relevant difference would concern HHLL elevation minus pump suction center elevation; corresponding gauge pressure would have a negative value in this case.
3. Even though max suction pressure could be defined at rated pump flow, I see it defined at no flow, which is higher than previous one by the friction loss in suction line. Posts by kkala use that concept. The question is what you specify in pump data sheets.

Edited by kkala, 30 April 2012 - 09:27 AM.

[DylanYang]

thanks everone, all of you really help me a lot and make me more understand Max suction

pressure and Max allowable shut off pressure.

i appreciate and grateful it.

thank you, suncerely.

[ankur2061]

DylanYang,

An important point to note during pump hydraulic calculations:

Avoid using gauge pressures for calculations such as suction pressure, NPSHa, and Discharge Pressure. This will ensure avoidance of minus signs in the equations for calculating these values. Only the static head (height) will have plus or minus sign depending upon the reference from the datum level of the pump suction and discharge nozzle.

Regards,
Ankur.

[kkala]

Points on post No 15 that could be useful.
-NPSHa is based on absolute (not gauge) pressure at pump suction nozzle, minus fluid vapor pressure, expressed as fluid head.
- Standard pump calculation sheets give gauge pressures for suction and discharge, at least here. This often occurs, http://www.cheresou...ction-pressure . However absolute pressures can be calculated instead, without problem. Of course both suction and discharge pressure have to be gauge or absolute in the pump data sheet (not mixed).
-Pump in reference uses gauge pressures, suction -1.97 psig, discharge 90 psig. If absolute pressures were used, these pressures would be 12.73 psia, 104.7 psia respectively. Which is more convenient depends on taste. Max shut off pressure (and resulting design pressure) may be more convenient in gauge units.
- Max suction pressure includes (pressure corresponding to) static head between HHLL and datum (centerline of suction nozzle), according to posts by kkala. Please inform whether friction loss in suction line is considered in max suction pressure calculation of pump data sheet (post No 13, comment 3).

Edited by kkala, 02 May 2012 - 01:15 PM.

[bhatt_ms]

Shut-off head is the discharge head developed by the pump when the outlet valve is completely closed(zero flow). THis can be calculated from the characteristic curve of the pump, by extrapolating the curve to zero flow. This is imp parameter for determining the design pr of the pump/ casing.