Performance of a liquid helium centrifugal pump for the TOPAZ superconducting magnet

Performance of a liquid helium centrifugal pump for the TOPAZ superconducting magnet T. Haruyama, T. M i t o , A. Y a m a m o t o , Y H Kajiwara*

Do=, K M a t s u m o t o * and

National Laboratory for High Energy Physics, KEK, Tsukuba, Ibarakl, Japan *Kasado Works, Httachu Ltd, Kudamatsu, Yamagucht, Japan

Recewed 30 July 1987 A centrifugal pump used for c~rculatmg hqutd hehum through a large superconducting magnet was tested The performance of the pump was measured during a magnet coohng test The pump operated well wtth atyptcal capacity of 400 dm 3 h -~ (Ap = 0 3 kg cm- 2) at 20000 rpm An analys~s of the test results ~s gtven

Keywords hehum, hehum pumps, superconducting magnets

Centrifugal pumps for circulating liquid or supercrltlcal helium have been developed and reported by many authors 1 s This type of p u m p is one of the major components of large superconducting magnet cooling systems Their excellent stability and reliability are necessary for long-term operation However, comparisons between the theoretical and experimental characteristics of such pumps can be found in only a few papers This may be due to the remarkable differences in fluid characteristics of liquid helium and water For the cryogenic system of the T O P A Z superconductIng magnet at T R I S T A N 6 7 a centrifugal pump was originally developed and installed s This pump is used to circulate two-phase low quality helium through the magnet, even if the refrigerator stops as a result of various problems, such as when the turbine trips The T O P A Z magnet requires 80 dm 3 h - 1 of forced helium flow as a mlmmum flow at a pressure drop of 0 01 kg cm 2 The pump was designed and built to have a typical capacity of 300 dm ~ h - ~ at 0 3 kg cm 2 of the delivery pressure r~se In thin Paper, we describe experimental results and analyses of the T O P A Z centrifugal pump

liquid helium) The T O P A Z superconducting magnet has a cold mass of 4 5 tonens of alumlnaum The cooling pipe of the magnet is 18 mm m d~ameter and 150 m in total length Details of the T O P A Z cryogenic system have already been reported in Reference 6

Pump description A cross-section and a photograph of the pump are shown in Ftgures 2 and 3, respectively The specifications of the pump are given in Table 1 The impeller has a diameter of 25 mm and comprises eight vanes (dmcharge angle 9") The impeller is a full-shrouded configuration with a shroud fitted by using the diffusion bonding technique fastened to the end of a shaft, the other end is connected to a driving motor The rotating mass of the shaft can be minimized by using a high strength titanium alloy The shaft is 10 and 27 m m in diameter and 240 mm long Since the motor IS operated at room temperature, vacuum

RES

Cryogentc system A schematic diagram of the forced cooling system of the T O P A Z superconducting magnet is shown in Ftgure 1 The two operating modes of the cooling system were built to circulate low quality helium through the magnet cooling pipe mode A is the refrigerator mode and mode B is the hquxd helium p u m p mode Under normal operation, the magnet is cooled using mode A When a problem occurs in the refrigerator, mode A is changed sequentially to mode B to maintain the coohng condition As shown in Figure 1, the p u m p and heat exchanger, which must ensure sufficient saturated liquid helium, are installed in the control dewar (storage capacity t000 dm 3 0011 2275/88/030157 04 $03 O0 ( 1988 Butterworth & Co (Publishers) Ltd

FROM/TO REFRIGERATOR

( ( {

[_HE PUHP

MAGNET

[ONTROL DEWAR

F0gure I

Schematic diagram of the TO PAZ magnet forced-coohng system P Pressure gauge F, flow meter

Cryogenics 1988 Vol 28 March

157

LtquM hehum pump for TOPAZ superconductmgmagnet T Haruyama et al

-

I

insulation and coohng water lines are also needed A small amount of hehum is passed into the shaft labyrinth to mlmmlze the conducted heat input through the shaft The evaporated gas ts also used for the static gas bearings A high frequency motor is used to achieve pump speeds of 12000-20000 rpm, the pump contains shaft sensors for speed monitoring The specific speed, whmh is one of the important parameters used to evaluate the pump, is defined as

CONTROL DEWAR TOP FLANGE

-VACUUMJACKET

¢c-

N~ = n Q ° 5 H - ° 75

216¢7

(1)

where n as the pump speed m rpm, Q as the capacity m m 3 man- l, and H is the head of hqmd in m (liquid helium m this case) The specific speed. N~, has been calculated m this pump as 130 at n = 20000rpm. Q = 5 × 1 0 - 3 m 3 rain- l (300 dm 3 h - 1) and H = 24 m A theoretical head of an ldeahzed centrifugal pump can be estimated using Eular's equation

COOLINGWATER PIPE .-,GAS BEt~RINGS MOTOR

Htla,.

= (u~ --

uZ)(2q) - ' + (c~ - c ~z)(2q)- i - (w 2 - w~)(2g)-1

(2)

--- TITANIUMALLOY SHAFT IMPELLER

Figure 2 Schematic diagram showing the cross-section of the hehum pump (total length ~ 2 m )

where Hth,. is the theoretical head of the pump (consldered to have an mfimte number of vanes), u~ and uz are the peripheral velocity of the impeller at the inlet and outlet radius, respectively, cl and c2 are the absolute velocity of the fired at the impeller inlet and outlet, respectwely, wl and wz are the relative velocity of fired at the impeller inlet and outlet, respectively, and 9 is the acceleration due to grawty If hqmd enters the impeller w~thout a tangentmi component, Eular's equation can be reduced to Hth, ~ = uZ29 - 1 _ u z Q ( 2 7 ~ r 2 b z

tan/~z)- 1

(3)

where r e is the outer radius of the impeller, b 2 i s the impeller width at the &scharge, flz as the discharge angle. and Q as the flow capacity Equation (3) represents the theoreUcal head of a centrifugal pump which has an infinite number of vanes The theoreUcal head of a pump which has a finite number of vanes can be obtamed as H,h = J ( z ) H , h , .

(4)

for f ( z ) = zs(zs + F r 2 ) - 1

where F is an experimental coefficient i-F = 0 6 ( 1 + 9/ 60) = 069 m this case], s is ( r ~ - r ~ ) / 2 , rl Is the tuner radms of the impeller, and z is the number of vanes (z = 8 m this case) The value of f ( z ) was calculated as 0 83

Ftgure 3 Photograph of the hehum pump

Table 1

Specifications of the hquld helium centrifugal pump

Inlet pressure (kg cm 2 G) Outlet pressure (kg cm 2 G) Capacity (dm 3 h -1) Speed (rpm) Specific speed [rpm(m 3 mln 1)05 m 0 751 Thermal efficiency Hydrauhc efficiency Hehum consumption for gas bearing (g s -1) Impeller diameter (ram) Number of vanes Discharge angle NPSH (ram) Shaft works (W)

158

(5)

Cryogemcs 1988 Vol 28 March

00 03 400 20 000 160 0 68 0 81 0 08 25 8 9~ <72 20

Operation After the T O P A Z magnet was cooled down to hquld helium temperature over five days and hquld helium (400 dm 3) was stored in the control dewar, a performance test of the pump was carried out The magnet coohng path was changed from refrigerator mode A to pump mode B, the pump was then started It was operated for several tens of hours under a variety of conditions in which the pump speed was vaned from 12000 up to 20 000 rpm while collecting test data on the performance of the pump

t/qutd hehum pump for TOPAZ superconducting magnet T Haruyama et al Results

relatmn as

and analysts

Ftgme 4 shows the test results In the form of the pressure rise across the pump versus the capacities at five pump speeds The dashed line in the figure shows the calculated relauon with Equation (4) at 20000 rpm The deviation lrom the dashed line may be due to a hydraulic loss m the pump, which ~s proportional to Q2 The calculated value was obtained using the following procedure From Equation (4) we can determine the head capacity relauon for a p u m p whmh has an impeller wath mght vanes The maximum values of H m and Qm were calculated to be 58 m and 876 dm 3 h - 1 respecnvely, m the case of water To apply these values to hqmd helium we had to take the differences m the propemes of the fluid into consideration The density and wscoslty of llqmd helium differ greatly from that of water (1/8 and 1/1000 the values for water respectwely) The kinetic vlscos~ty v = r//p IS 1/36 that of water The maximum head of this pump for hqmd hehum was calculated to be 0 72 kg c m - -' pressure rise through the pump, the max> mum capacity was calculated to be 876 dm s h a From the experimental results of Q - A p , we can estimate the hydrauhc losses of this pump and all the fncnon and dlffusmn losses, Ap~a, by a simplified equanon

Ap = Ap,h

-

K,Q.'-

-

K2(Q.

-

-

Oj 2

(8)

= aQ e + bO +

We determined constants K~ and K 2 from an actual Q - k p curve by selecting several points on the curve Ftgme 5 shows the calculated results of the friction losses, shock losses and hydraulic efficiency (defined as r/h = H,IH,h = Ap'Apth) At low capacity, the shock losses are greater than the frlctmn losses, for example, the ratio of the fnctaon losses to shock losses is calculated as 0 19 at Qd = 300 dm~ h ~ (design specification) One reason why the fnctaon loss as so small may be due to the low wscoslty of liquid helium The best efficmncy can be obtained for Qn= 500dm~h ~ At thin point, the ratio between the fnctmn losses and shock losses increases to 0 66 In Ft,~ure 6 results are shown in the form of head coefficmnt vet sus flow coefficmnt parameters Thin form is conventionally used for a description of the turbomachmerv performance ~ Presentanon of the data using

10 i

[

I

l

I

Ii°

(6)

A p t d = K , Q2

The fractmn losses generated when hquld passes through a sucUon nozzle and Impeller channel are Included in Aped The diffusion losses at the impeller channel and discharge nozzle are also included The shock losses at the ampeller entrance and exit can be expressed by the following equation Ap, = / ~ 2 ( Q -- Q,)2

(7)

08 t

~'4b ~ LubS

~

-

\

06

',

06

~.

o~-w

L.U

Q:: CC]

At a capacity Q, (shockless), the direction of flow agrees with the vane angles at both entrance and discharge Thus, Equanon (7) represents a square parabola with ~ts apex at Q, Therefore, we can now express the Q - Ap

I

I an

Id

200

°'s I

400

600

800

CAPAEITY

08

l

I

'

I

~

I

I

I

z ~J

1000

O (L/H)

F~gure 5 Charactermtm curve obtamed by subtractton of hydraulic losses from theoretmal pressure rme, Apt h The hydraulic efficiency, t/h IS also shown Pump speed = 2 0 0 0 0 rpm

'

*.. \

\

08

x. N

\

\

I

I

\


06-\

\ \

F-k~J k.J

\

04--

\

~-

Or"

u_ g

¢/3

\

~02--

,,

t-r"

_.__

\

\

\\

04--

\

#o'\

&~

•

<~

\

\

\x

~o2-

\

~°~ o oo\ \

_J 0

I 200

,

I zOO

,

1

i

\\

\\

\

0

I

\\

i\\, I

600

800

EAPAEITY

Q (L/H)

F i g u r e 4 Experimental results of head-capacity performance Theoretmal curve for an operating speed of 2 0 0 0 0 r p m 20000rpm • 18000rpm ~, 17000rpm, ~, 16000rpm 12000 rpm ~ design specffmatmn point

1000

0 --

005

O1

015

EAPAEITY COEFFIEIENT , © []

Ftgure 6 Head coefficient capacity coefficient relation of the hehumpump - Theoretmalrelatlonwhenthedmchargeangle of the i m p e l l e r ~ s f l 2 - 9 ,expenmentalrelatlon(f12=41 ) • design spectftcat~on point

Cryogenics 1988 Vol 28 March

159

Ltqutd hehum pump for TOPAZ superconducting magnet T Haruyama et al

these variables normalizes test results obtained at various speeds and pressures The head coefficient, ~b, and the flow coefficient, tp, are defined by 4 ¢ = Ap(pu 2)-1

(9)

E

03 "T

¢I_ <3

- 500 "~ ,._1

,,,0

- 40O

and q~= Q(6OuzA2) -1

(10)

where Ap is the pressure rise across the pump, Q is the capaoty of the pump, p ms the fluid density, and A z is the exit area of the impeller All data were obtained at a temperature of 4 5 K In Ftgure 6, the solid hne m&cates the (p-~, relation for this pump, which has an impeller discharge angle of#2 = 9 ° Experimental results, however, agree well with the dashed line which indicates a ~b-g, relation at flz = 4 1° The tangent of the discharge angle indicates the ratio between the peripheral velocity of the impeller and the mendional velocity at the maximum capacity point It can, therefore, be considered that a large difference in the viscosity makes the effective d~scharge angle smaller than that of the actual impeller The design specification is indicated in the figure by a closed orcle The ratio of useful fluid power, W, to cryogenic loss, Q , - Qs, md]cates the thermal efficiency9 7, = W(9_, - gs)- 1

(11)

where Q, is the total heat input to the control dewar when the pump is operated and Qs is the total heat input when the pump is shut off The fluid power is given by W = QAp

Conclusions A hqmd hehum centrifugal pump for the T O P A Z super° conducting magnet was tested Measurements show that

Cryogenics 1988 Vol 28 March

-

i11 ¢,e"

300

>I.-Q-

~o

200 ~

I.M ¢',eO-

1 100

-50

0

100 NPSH (mm)

I

20O

Figure 7 Cav=tatton charactensttcs of the centrifugal pump for a pump speed of 20000 rpm and Q ~ 430 dm 3 h -1

the typical performance of the pump was a capacity of 400 dm 3 h - 1 (Ap = 0 3 kg cm -2) at 20000 rpm Experimental performance results agreed approximately with those predicted theoretically The measured thermal efficiency was 0 68 and the maximum hydraulic efficiency was 0 81 It was found that the pump performance was satisfactory as a hquld hehum circulator in the cryogenic system for the T O P A Z superconducting magnet

(12)

where Q is the capac]ty and Ap is the pressure rise through the pump The bod-off rates were measured by flow meters at room temperature, Q, = 12 4 W, Qs = 6 9 W and W = 3 73 W at a pump speed of 20000 rpm (Q = 457 dm 3 h -1 and Ap = 0 3 kg cm -2) The result shows a thermal efficiency of r/t = 0 68 The net posltwe suction head (NPSH) is defined as the static hquld head in the vessel above the pump centre line minus the losses of head in the suction pipe when boding hqmds are pumped from a closed vessel The required N P S H can be calculated by using Thoma's constant as 1 25 m of the llqmd helium head in this pump Ftgure 7 shows the test result for performance under low N P S H conditions The pump was operated at 20000 rpm and 430 dm 3 h-x The performance of this pump did not change until N P S H was ,,~72 mm, and abrupt changes in the pressure rise and capacity (a feature of the cavitation) were not observed, even at negatwe N P S H values

160

r'.e"

0

Acknowledgements The authors wish to thank H Suzuki, Y Kondo, M Kawa], M Klmura and H Yamaoka of KEK, Y Ibarakl of Telsan Co Ltd, and I Kawamura of H]tach], Ltd, for their contributions to these tests

References 1 Daney,D E, Ludtke, P R and Smdt, C F Adv Cryog Eng (1968) 14 438 2 Stxsmlth, H and Gtarratano, P Rev Sct lnstrum (1970) 41 1570 3 Lehmann,W. and Mlnges, J Adv Cryog Eng (1983) 29 813 4 Swift,W., Sixsmtth,H. and Schlalke,A Adv Cryog Eng (1981)27 777 5 Cairns, P.NH and Brassmgton,D J Cryogemcs (1976) 16 465 6 Yamamoto, A, Kic~mt, H, Klmura, N, Inoue, H., Yamaoka, H, Haruyama, T, Mtto, T., Araoka, O., Tndano, M., Suzuki,S, Kondo, Y, Kawat, M, Din, Y and Hlrabayashl,H Jpn J Appl Phys (1986)25 L440 7 Din, Y, Mlto, T, Haruyama, T, Kimura, N, Araoka, O, Tndano, M., Suzuki, S., Kondo, Y., Kawat, M, Yamaoka, H, Ktchtmt, H., Yamamoto, A. and Awata, Y Proc 1CECll (1986) 424 8 Din, Y, Yamamoto, A., Awata, Y and Wada, O H~tacht Revww (1985) 34(3) 127 9 Lue,J W, Miller, J R, Walstrom, P.L and Herz, W Adv Cryog Eng (1981) 27 785