Tom & others:
Different types of pumps and various conditions can cause confusion on the
effects that are produced. Rather than going back to the engineering
books....I refer to some basic pump principles to keep things straight:
- -the GMC water pump (and all automotive water pumps) are centrifugal pumps.
- -the lowest HP input for a centrifugal pump is at "shutoff" (flow restricted
to zero flow)
- -the highest HP input for a centrifugal pump is at its maximum flow. This
is easier to understand by thinking of "work" output. A centrifugal pump
does the most "work" when it moves the most water, and the most water is
moved at it maximum flow. Flow typically increases with RPM until it
reaches its maximum flow point, then flow begins to decrease as RPM
continues to increase. This area of decreasing flow with increasing RPM is
called runout. It is caused by cavitation of the impeller.
- -system friction affects work out not work in.
- -work out divided by work in gives efficiency.
- -A positive displacement pump (like a lube oil pump) would provide different
results. It requires the maximum HP input at shutoff (deadheaded) and will
break something if there is not a bypass or relief valve. Flow on a
positive displacement pump will increase with rpm until the point of
cavitation.
CONCLUSIONS:
- -If a system such and the GMC cooling water system (engine flow path, hoses,
radiator, etc) does not have enough restriction, then with high enough RPM
the pump with not perform efficiently. This probably occurs without a
thermostat or some other restrictor.
- -The efficiency of the pump is affected by the impeller configuration,
impeller clearance, and RPM. Unlike the GMC, most industrial centrifugal
pumps have a very precise pre-set or adjustable impeller clearance and run
at a constant RPM.
- -Putting a restrictor in the outlet of the GMC water pump have no effect on
HP required at low RPM (idle). -At increasing RPM, the restrictor will
generally decrease the HP required to drive it, because the pump is moving
less water and doing less work. It would only increase HP input required
(if compared to a pump with no restrictor) when operating at a high RPM by
producing more flow then a pump with no restrictor which reached runout
causing flow to drop off.
- -The correct restrictor would "tune" a pump for maximum efficiency and
performance at a given rpm band. -The restrictor could vary from case to
case based upon pump impeller diameter, number of blades, configuration of
blades, impeller to casing clearance, radiator cross section configuration,
etc.
- -To develop a pump curve, testing would be required to measure all of the
parameters and know how the complete system performs. This would be
difficult because of the need to measure flow, pressure, and HP imput and do
it at various RPMs.
- -Larry Price (another engineer type....for rotating equipment @ nuclear
power plants)
73 Painted Desert
>Date: Fri, 24 Dec 1999 23:17:39 -0500 (EST)
>From: Tom Warner
>Subject: Re: GMC: water pump restrictions
>Haven't gone back and read my engineering books but intuitively I think
that
>putting a restrictor in the outlet of the water pump would increase the HP
>needed to push fluid through the sytem, ie; increasing the friction in the
>system. The only way to reduce the HP used by the water pump would be to
>change the pully sizes that drive it and lowering the RPM of the pump at a
>given engine speed.
>All the restictor is going to accomplish is reducing the flow thru the
radiator.
Different types of pumps and various conditions can cause confusion on the
effects that are produced. Rather than going back to the engineering
books....I refer to some basic pump principles to keep things straight:
- -the GMC water pump (and all automotive water pumps) are centrifugal pumps.
- -the lowest HP input for a centrifugal pump is at "shutoff" (flow restricted
to zero flow)
- -the highest HP input for a centrifugal pump is at its maximum flow. This
is easier to understand by thinking of "work" output. A centrifugal pump
does the most "work" when it moves the most water, and the most water is
moved at it maximum flow. Flow typically increases with RPM until it
reaches its maximum flow point, then flow begins to decrease as RPM
continues to increase. This area of decreasing flow with increasing RPM is
called runout. It is caused by cavitation of the impeller.
- -system friction affects work out not work in.
- -work out divided by work in gives efficiency.
- -A positive displacement pump (like a lube oil pump) would provide different
results. It requires the maximum HP input at shutoff (deadheaded) and will
break something if there is not a bypass or relief valve. Flow on a
positive displacement pump will increase with rpm until the point of
cavitation.
CONCLUSIONS:
- -If a system such and the GMC cooling water system (engine flow path, hoses,
radiator, etc) does not have enough restriction, then with high enough RPM
the pump with not perform efficiently. This probably occurs without a
thermostat or some other restrictor.
- -The efficiency of the pump is affected by the impeller configuration,
impeller clearance, and RPM. Unlike the GMC, most industrial centrifugal
pumps have a very precise pre-set or adjustable impeller clearance and run
at a constant RPM.
- -Putting a restrictor in the outlet of the GMC water pump have no effect on
HP required at low RPM (idle). -At increasing RPM, the restrictor will
generally decrease the HP required to drive it, because the pump is moving
less water and doing less work. It would only increase HP input required
(if compared to a pump with no restrictor) when operating at a high RPM by
producing more flow then a pump with no restrictor which reached runout
causing flow to drop off.
- -The correct restrictor would "tune" a pump for maximum efficiency and
performance at a given rpm band. -The restrictor could vary from case to
case based upon pump impeller diameter, number of blades, configuration of
blades, impeller to casing clearance, radiator cross section configuration,
etc.
- -To develop a pump curve, testing would be required to measure all of the
parameters and know how the complete system performs. This would be
difficult because of the need to measure flow, pressure, and HP imput and do
it at various RPMs.
- -Larry Price (another engineer type....for rotating equipment @ nuclear
power plants)
73 Painted Desert
>Date: Fri, 24 Dec 1999 23:17:39 -0500 (EST)
>From: Tom Warner
>Subject: Re: GMC: water pump restrictions
>Haven't gone back and read my engineering books but intuitively I think
that
>putting a restrictor in the outlet of the water pump would increase the HP
>needed to push fluid through the sytem, ie; increasing the friction in the
>system. The only way to reduce the HP used by the water pump would be to
>change the pully sizes that drive it and lowering the RPM of the pump at a
>given engine speed.
>All the restictor is going to accomplish is reducing the flow thru the
radiator.
