limestone slurry pump oil and gas

Minimum Flow

Minimum Flow : THERMAL and STABLE

A minimum required flow rate should be specified. It is important to determining if a minimum flow bypass is required for the selected pump to avoid pump overheating and mechanical damage.

1- Minimum Continuous Stable Flow:

In establishing the NPSHA, the purchaser and the vendor should recognize the relationship between minimum continuous stable flow and the pump’s suction-specific speed. In general, minimum continuous stable flow increases as suction-specific speed increases

lowest flow at which the pump can operate without exceeding the vibration limits imposed by this International Standard {API 610}

Item 1 in the below photo defines the allowable operating region. Allowable operating region is specified by vendor. The lowest point of this region is called minimum continuous stable flow. This point shall not be greater than our required minimum flow which is specified in the mechanical data sheet. If pump can pass the tests as per table 8 and 9 of API 610, the specified min. continuous Stable flow by vendor is true.

Minimum Continuous Stable Flow is the same the lowest point of allowable operating region.

The lowest allowable pump’s flow where the pump operates with the lowest vibration (within the allowable vibration range).

Where a pump must operate under two or more sets of conditions, it shall be designed for satisfactory recycle to overcome minimum flow conditions is required. Seller shall propose in the bid.

When a pump operates at low capacities, the fluid is subject to a temperature rise which is proportional to the difference between the brake horsepower absorbed by the pump and the fluid horsepower developed by the pump. The vapor pressure of the fluid is accordingly increased and as the flow rate is throttled, a point will be reached where vaporization occurs in the pump resulting in cavitation and ultimately the pump may become vapor bound and inoperative. When process considerations require extreme throttling, a minimum flow bypass must be installed, preferably from the pump discharge back to the suction vessel. While the minimum safe continuous flow can be calculated, the following guidelines are offered by pump manufacturers:

Single stage pumps, bypass = 15-25%

Multistage pumps, bypass = 25-35%

Worthington suggests 30 GPM/100 horsepower

Exon Mobil:Pumps in clean liquid services shall be suitable for continuous operation at 30% of the rated capacity given on the pump data sheet. The contractor shall advise the Owner if any service requires a circulation system for continuous operation at the 30% level.”

2- Minimum Continuous Thermal Flow:

lowest flow at which the pump can operate without its operation being impaired by the temperature rise of the pumped liquid {API 610}

Calculation of min. continuous thermal flow: {Sulzer}

Qmin = (P x 3600) / (ρ.c.∆t)

Qmin = m3/hr (min. continuous thermal flow)

P = power (kw) (at or near shutoff)

ρ = density of the liquid (kg/m3) (i.e. water = 1, HC = 0.3 – 0.6)

c = specific heat of the fluid (kj/kgk)

∆t = permissible heat up, ºC

More Instruction: {foster wheeler Design manuals}

In general, a conservative approach is to supply pump systems with a system to prevent the pump from operating below its recommended minimum flow:

Temperature Rise

Radial Bearing Loads

Axial Thrust

Prerotation

Recirculation and Separation

Settling of Solids

Noise and Vibration

Power Savings, Motor Load

Ten Ways to Prevent Low Flow Damage in Pumps:

Continuous Bypass

Multi-Component Control Valve System

Variable Frequency Drive

Automatic Recirculation Valve

Relief Valve

Pressure Sensor

Ammeter

Power Monitor

Vibration Sensor

Temperature Sensor

Comments:

The vendor shall state on the individual data sheet the min. continuous thermal / stable flow rate for all pumps.

Min thermal flow shall not be less than below calculation.

Q min = (P x 3600) / (ρ.c.∆t)

Q min = m3/hr (min. continuous thermal flow)

P = power (kw) (at or near shutoff)

ρ = density of the liquid (kg/m3) (i.e. water = 1, HC = 0.3 – 0.6)

c = specific heat of the fluid (kj/kgk)

∆t = permissible heat up, ºC

Continuous stable flow shall be less than our identified min. flow.