Harwood
Engineering Company, Inc.
Pipless (Pulseless) Pumping
by William C. Newhall
Discontinuous or fluctuating flow rates frequently present process control
problems, particularly where high and often even moderate pressures are
involved, due principally to the compressibility of the high pressure fluid.
Herein are described arrangements of hydraulically driven pumps that overcome
this problem and that have been applied to systems both of gases and liquids,
ranging in pressure from a few thousand to 200,000 psi. The problem is
analyzed and its solution described.
In many manufacturing processes, it is very desirable to have a constant
flow of fluid from a pump free from both "ripple" and interruptions (1).
In the chemical processing field, for example, and particularly in polyethylene
production (2), where chemicals must be mixed in correct proportions under
pressure for a product to result, the ripple or interrupted delivery from
normal pumps causes some problems in process control. Another field in
which ripple or an interrupted delivery might cause problems is in the
application of the use of high pressure fluids to jet cutting applications
(3). This is a new field in which everything from delicate papers, too
weak to be cut by conventional means, to rock cutting for tunnel building,
has been done. Successful jet pressures have been used from about 50,000
psi to 200,000 psi. Definitely showing promise in the development stage
is the application of high pressure to extrusion (4), particularly extrusion
of brittle materials. High pressure is used to support the die during the
extrusion, to supply the force of extrusion and in "liquid to liquid" extrusion
to support the extrudent. The pumping system has to maintain control of
the pressures in the system regardless of the changes in volume involved.
It would be disastrous to a given run to start the extrusion and reach
a "pip" in the pressure supply when the process requires a steady flow
of fluid to keep the work going. With the pressure lost, not only would
the extrusion abruptly stop (and probably, the materials being brittle,
break the billet), the die might break without its normal support pressure.
Pipless Pumping System
The pipless pumping system (5) provides solutions to all of the above problems.
It will provide a steady, essentially uninterrupted flow, regardless of
process pressure. Not only can the Harwood system provide pipless flow,
but the flow rate is readily controlled by signals, responding to any process
variable or combinations thereof that can be put into an industrial controller.
The pips in the conventional pumping systems, including the conventional
Harwood hydraulically driven pumps, are caused by the compressibility of
fluids at high pressure. Compressibility becomes an increasingly difficult
problem as the pressure increases; for example, a mixture of water and
Zerex has a compressibility of about 20% at 200,000 psi. The pip problem
is further complicated in slider-crank type mechanisms due to their inherent
sine characteristic.
Figure 1 shows the delivery pattern to be expected
from a hydraulically driven single-acting pump without the pipless feature.
This clearly shows that most of the time in a pumping cycle is spent in
piston retraction and in taking up the compressibility of the fluid. Figure
2 shows the delivery of a double-acting intensifier pump also without
the pipless feature. Here, the time of no delivery is much less, being
mostly the time required for the piston to take up compressibility.
Harwood has developed pipless pumping systems for both double-acting
and single-acting intensifiers. Each as its place. When the compressibilities
are relatively small, as with most liquids, generally the single-acting
intensifier pipless system is the one of choice. Gas systems which have
considerably larger compressibilities are more conveniently handled with
doubleacting intensifier systems. Since the principle involved in either
case is the same, and an understanding of one system makes the other obvious,
only the single-acting system will be described at length in this paper.
Method of Operation
The pipless pumping system provides, by means of valving on the drive side
of the intensifiers, a method whereby the retraction and then the compression
phases of the stroke occur so that they do not interrupt the delivery of
the high pressure fluid. At least two intensifiers are necessary. While
intensifier I delivers high pressure fluid at a controlled rate, intensifier
II concurrently retracts and then advances to take up the major portion
of the compressibility to a pressure very slightly below that of delivery.
It dwells momentarily until the other intensifier completes its delivery
stroke. Valve operators now function to reverse the piston of intensifier
I and to boost the drive pressure of intensifier II to effect delivery.
This is illustrated in Figure 3, where on a common
time coordinate, the flow from intensifiers I and II are shown separately
and their flows superimposed to show the resultant pipless delivery.
Referring to Figure 4, the basic elements of
the system are the intensifiers I and II, the drive pumps PF-1 and PF-2,
the intensifier selecting valves A-1 and A-2, the reversing valves A-3
and A-4, the differential pressure regulating valve U, the pressure compensated
flow control valve T-3-C and the back pressure valve R. As shown, intensifier
I is delivering and intensifier II is precompressing. At all times, PF-1
drives the intensifier which is delivering. During the intensifier retraction
phase, the intensifier being retracted is driven down by the back pressure
trapped in the system by back pressure relief valve R; contributing to
the flow in the retraction phase of the stroke is the flow from PF-2 and
any overflow from compensating valve C. This assures that the retraction
will be done relatively quickly.
When the intensifier starts to go into the predelivery compression phase
of its pumping cycle, only the flow from pump PF-2 — as regulated by the
differential pressure regulating valve U — is used to make the pre-delivery
compression. The pressure compensated flow control valve T-3-C controls
the flow rate of the high pressure fluid by controlling the flow of the
delivery drive pump.
The intensifier selection valves A-1 and A-2 select which intensifier
will deliver first and then keep the intensifiers in sequence. The intensifier
reversing valves A-3 and A-4 allow the pistons in the intensifier to move
up or down. All four of the directional control valves A-1, A-2, A-3 and
A-4 are controlled by limit switches on the intensifiers. When the intensifier
making the delivery stroke (I in Figure 4) reaches the end of its delivery
stroke a limit switch signals this fact to valves A-1, A-2, and A-3. The
shifting of valves A-3 and A-1 starts intensifier I into its retraction
phase; the shifting of valve A-2 starts intensifier II into its delivery
phase. All of these valve shifts occur simultaneously. When intensifier
I reaches the end of its retraction stroke, valve A-3 shifts, allowing
intensifier I to start its pre-delivery compression. This constitutes the
first half cycle. The second half is identical except that the functions
of intensifiers I and II and of valves A-3 and A-4 are interchanged.
Differential Pressure Regulating Valve
The key to limiting the pre-delivery compression so that the intensifier
in pre-delivery compression does not start to discharge prematurely is
the differential pressure regulating valve U (see Figure
5). The regulated pressure, to be maintained 50 psi, more or less,
below the delivery drive pressure, is brought to the spring side and to
the pressure port of valve U, normally open. The delivery drive pressure
is brought to the non-spring side of the valve and is of sufficient magnitude,
applied over the face of the valve spool, to close the valve. A force balance
is established between the delivery drive pressure on one side of the spool
and the regulated pressure and the adjusted spring on the other. If the
regulated pressure increases to an arbitrary differential, set by the spring,
below the delivery drive pressure, the valve opens and enough pump fluid
is spilled off to close the valve and re-establish the force balance. This
self-relieving feature of the valve U is very important because it means
the hydraulic drive will be automatically responsive to any high pressure
changes in the output of the system. In particular, a sudden decrease in
the process pressure will not cause the unit under precompression to start
delivering prematurely; on the contrary, the delivery drive pressure on
the non-spring side of valve U will drop, opening the valve and relieving
the regulated pressure to a level below the new delivery drive pressure;
this also lowers the pressure of the precompressed charge in the precompressing
intensifier. A new force balance is set up on valve U at the new lower
drive pressure.
Flow Control
The control of high pressure flow rate is obtained by accurately controlling
the flow of the delivery drive pressure to the delivering intensifier.
Refer to Figure 6. The throttle valve T-3 will have
a pressure drop across its orifice. The valve C, a spool valve, is normally
closed, with a relatively light spring in the spring cavity. Oil pressure
directly from the pump PF-1 is sensed on the non-spring side of the valve;
oil pressure downstream of the throttle T-3 is sensed on the spring side.
A force balance is created between the higher pressure oil from the pump,
as sensed on the non-spring side of the valve, and the oil pressure downstream
of the throttle T-3 plus the spring force on that side of the valve.
Should the pressure drop caused by the orifice of the throttle T-3 be
large enough, the force exerted on the bottom of the valve spool by the
pump pressure will be greater than the force exerted on the top of the
spool by the spring plus the throttled pressure, allowing it to open and
spill across the excess flow. By thus limiting the pressure drop allowed
across the throttle, the flow of drive fluid is made a function only of
orifice size. By placing either an air motor with positioner or a more
sophisticated electronic-servo drive on the throttle valve, a rigorous
control of the high pressure flow is obtained which is sensitive to whatever
parameter is fed back into the air motor or servo-mechanism.
Also shown on the schematic are the necessary flow relief valves (RR-1
and RR-2) for the protection of the drive pumps, and an operator's relief
valve (RRC) permitting the operator to limit the system to a lower than
rated working pressure. The hydraulic spool valves U and C are shown with
their biased restrictors T-4-CH-7 and T-5-CH-8 respectively; these are
necessary to dampen the response of these valves in purely transient conditions.
The intensifiers I and II are equipped with biased restrictors T-1-CH-3
and T-2-CH-4 to stop the possibility of hydraulic surge while the intensifiers
are retracting.
Fluid conditioners — the filters (FH-1 and FH) and the heat energizer
(HE) — to keep the oil clean and cool, are also illustrated. All of these
hydraulic components are basically standard 3,000 psi fluid power components.
Conclusion
The pipless pumping system is a unique means of solving a rather difficult
flow control problem. First of all, the flow is essentially uninterrupted
and the design of the process control system is simpler than with conventional
pumping equipment. Being able to control the flow of the delivery by means
of the relatively low drive pressure flow, it is possible to employ the
proven and more accurate 3,000 psi control valves. It is, for example,
possible to get pressure compensation built into the drive pressure flow
control valve, a feature impossible with high pressure flow control valving.
Also, because the control valving is standard, it is easily adaptable to
many different transducers, which respond to variables on the delivery
side of the system.
The system described has proved very successful in providing steady
flow of fluid. While for the purpose of explanation the combined flow pattern
of Figure 3 shows pips attendant to intensifier reversal, in actual practice
systems can be adjusted so that it is impossible to discern the pips on
a strip chart record of the flow.
In the preparation of this paper the author wishes to express his appreciation
for the interest and constructive criticism of Mr. Donald H. Newhall and
Mr. Leonard H. Abbot.
References
1. Newhall, D.H., Hydraulically Driven Pumps,
Ind. Eng. Chem., 49 p. 1953 (Dec. 1957)
2. Kresser, T.O.V., Polyethylene, p. 67-, Reinhold Publ. Co.,
New York, 1957.
3. Jacques, M.E. and Kurko, M.C., Machining with high velocity hydraulic
jets, Hydr. and Pneum., 24, No. 2, pp. 56-9 (Feb. 1971).
4. Pugh, H. Ll. D., Hydrostatic extrusion, N.E.L. Report No.
416, May 1969 and Paper 18. Ninth Commonwealth Mining and Metallurgical
Congress, 1969, issue by the National Engineering Laboratory, East Kilbride,
Glasgow, or Office of the Institution of Mining and Metallurgy. This contains
a bibliography of 43 items, including several by Vereschchagin, L.F. and
his collaborators in Russian.
5. Newhall, D.H., Hydraulic systems for the development of a continuous
flow of liquids or gases at higher pressure, Pat. No. 2,463,552.
Newhall, D.H., Systems for delivering a continuous and steady flow
of a compressible fluid at high pressure, Pat. No. 2,819,835.
Maglott, G.F., Pipless pumping, Pat. No. 3,077,838
Maglott, G.F., Differential pressure control for pipless pumping,
Pat. No. 3,201,031
Harwood Engineering Company, Incorporated – © June, 1999
455 South Street, Walpole, Massachusetts 02081
phone: 508-668-3600
fax: 508-660-2276
http://www.ultranet.com/~harwood/
email: harwood@ma.ultranet.com
