header tank cap failure - datalogging results

[Rob Walker]

Raceline has run the Zetec and Duratecs for years without any expansion /header tank

[Colin Mill]

Hi Bri

 

Yes, that is why I'm keen to get rid of the Rover 'pressure' cap and have a Stant cap. From the earlier comments about pressures of about 1 Bar I guess the Rover cap is supposed to vent at about 15psi but who knows??

 

With a Stant cap on an 'airless' system the Stant cap lifts to release water as soon as the water pressure gets to 13psi (or whatever the rating of the cap) which, as you say, with the incompressibility of water happens as the water starts to heat up.

 

Just to do the sums, the cubic expansivity of water is 210E-6 per deg C and the cubic expansivity of aluminium ( 3 times the linear expansivity) is 70E-6 per deg C. So the effective expansivity of water in ali is = (210 - 70) E-6 = 140 E-6.

 

Taking the system capacity as about 5 litres and the temperature rise as typically 80 degrees gives us 5000 x 80 x 140E-6 = 56cc of water expelled via the cap.

 

The expansivity of ethylene glycol is 570E-6 per Deg C so with pure antifreeze you would expect more like 200cc expelled and I guess the mixture we use will give us something in between these values - say 125cc.

 

At the moment I have an airspace of about 200cc in the top of my header tank and this certainly gets compressed by about a factor of two as it warms up.

 

Edited by - Colin Mill on 28 May 2008 15:59:45

[Bricol]

Just to add a little more interest/waste some more time reading, take a look at this blokes thoughts and ideas on cooling fitting a K-series into a Spitfire here.

 

So Colin - I may have mis-read your ideas - more concerned with the unreliability of the Rover cap?

 

I must admit to being bemused by talk of unreliable Rover caps - maybe I've been lucky with all our Rover engined vehicles. Caterham is still on it's original, 7 yr old cap and runs fine - modified with PRT stat tho in all temps, speeds (low geared too - so high revs for long time on European motorways to get south as quick as possible).

 

I wonder if any other manufacturers caps would fit? Never had any hassle with similar designs on Renaults either.

 

The trials Dutton xflow, spending long periods at high revs too (really, really low geared - valve bounce in top is at 57mph!) and long periods idling, runs the old fashioned systems reliably (as it looks like Raceline do with more modern engines) - using Stant caps bought from Walmart. Only time one failed was when I drove over it! Supposed they vent at 16psi, slightly up on the original 70's 13psi.

 

So, maybe the easiest option is a different tank using a old style cap - the integrale uses such a system and I use a Stant cap on that too. That too is simply tee-ed into a hose from the water pump on the back of the block - no actual coolant flow through it. Tends to find it's own level and stay there as it vents to the ground. Temps stay nailed at 90 crawling, blasting, or embarrasing Saxos while towing a tonne and a bit trailer. Uphill. On the Cat and Fiddle road.

 

Maybe I just take cooling systems a little too simplistically - add coolant, remove air -test. Keeps cool in all conditions - job done. Doesn't keep cool - check system, increase rad size/air flow/water flow - check again. Never really gave much though to pressure fluctuations in use, as pressure will change as the coolant warms up/down, engine revs rise/fall etc.

 

I'll await more results from Peter with interest - but I probably won't take any action on them as I seem to have the same problems . . . yet . . .

 

Bri

[Colin Mill]

I must confess I like nice simple cooling systems of the old-fashioned kind as they never give any trouble. My 1963 Massey Ferguson goes from one year to the next without a problem - look in the rad to see that the level is right (it's never wrong!) and go. Flush it out every few years and re-fill. It has a 13psi cap and an open vent pipe (like everything else from the 1960s)

[Colin Mill]

I'm thinking that there may be another solution to this problem but to see if its a goer I wondered what people thought was the perfect way for the pressure to behave.

 

Would it be ideal if the pressure rose quickly almost as soon as the engine was started to, say, 13psi and maintain this value until some time after the engine was stopped - say 5 minutes? Or should it only be pressurised after some temperature is reached?

 

Any thoughts gratefully received.

[Malcolm]

Anyone else feeling dizzy ?

[CageyH]

I am actually going through replacing the header tank now.

I'll be fitting a 100mm diameter (Similar to the Apollo) x 200mm tall vertical tank. This takes a standartd cap. The question is, should I fit an uprated pressure cap, or stick to a 1 bar rated cap?

 

I have seen some nice Stant caps rated at about 19 PSI with a built in lever to reduce the pressure before you remove the cap for maintenance. Are these any good?

 

Only dead fish go with the flow....!

[Colin Mill]

In going for an uprated pressure cap I guess there are two considerations. Firstly, what increase in boiling point will the higher pressure give.? Looking at the tables I have for water the 1bar pressure raises the boiling point from 100C to 120C (antifreeze/water mix boils at a above 100C but the lift in boiling point will still be about 20C). Going from 1 bar to 1.7 bar (about 25psi) only lifts the boiling point another 10C. So I guess the question is will that 10C be significant

 

The next thing is will the system stand the extra pressure. My guess is that the big diameter hoses are the things that are the limiting factor. It's easy to work out the stresses involved - the largest stress is the one trying to split the pipe down its length (the stress trying to rip it along a circumference is only half as big). Anyway, a 1.25 inch diameter pipe at 20psi has a force of 25 lbs trying to pull apart the two halves of the pipe for every inch of length. So as a crude test you could simply try pulling a 1 inch off-cut of pipe apart with 25 lbs (plus safety factor) between two rods slid into the tube. My guess is that any half decent rad hose will cope with 20psi quite easily.

[CageyH]

Good point.

I have new silicon hoses on the car, and I would guess that the weak points will be the joints. The car will probably not reach 19 psi onthe cooling system, as it has never blown a Rover cap before.

 

I am still trying to locate a suitable header tank.

 

Only dead fish go with the flow....!

[Colin Mill]

Are you looking for an aluminium one?

[CageyH]

Not really, but having looked long and hard for one to fit in with the inboard suspension, I have gone for this one.

 

This was the cheapest place I could find it as well...

It looks like it will do the job quite nicely, as it is slightly smaller than an Apollo tank, so at least I know it will fit.

 

 

 

Only dead fish go with the flow....!

[Colin Mill]

I'm going to see what Evalution want to make up a special - since the stock ones are about 65 quid I guess it will be serious 😬

 

I'm not jumping just yet however as I want to have a play with the idea of using a small pump (a screen washer pump may well be enough) in conjunction with a pressure transducer and the inevitable microcontroller to pre-pressurise the system. I know its overkill but it could be a very light solution.

[ECR]

I have a tank (very similar to the one cagey has linked to) if you are interested in trying it. If it works we can agree a sensible price. I also have an uprated Stant cap and a filler neck for welding on to a purpose made tank ....

 

Ex Chairman Roger

[CageyH]

ECR,

 

Have you tried the tank?

Any thoughts on it's suitability?

 

Only dead fish go with the flow....!

[Colin Mill]

I also have an uprated Stant cap and a filler neck for welding on to a purpose made tank ....

Hi Roger

 

I would be interested in this as I might well decide to make my own tank for experimentation.

 

BTW does anyone know the diameter of the water pump impeller and some idea what the ratio of engine revs to pump revs is? I ask because I would like to do the sums to calculate the possible pressure differential at various revs. I have been surprised at how high the pressure is coming out of small centrifugal pumps (like some screen wash pumps) so perhaps this could be a factor in Peter's measurements.

 

I'm starting to think that the idea of removing the water stat and using an automatic radiator blind might not be such a silly idea after all.

 

Edited to say that from pictures etc I reckon the pump is about a 60mm diameter rotor turning at engine rpm. So at 4000 rpm this would have a stall pressure of about 0.7 bar.

 

 

Edited by - Colin Mill on 31 May 2008 14:19:41

[Mr Locust]

I suspect this thread has done a major swerve away from the intent of Peter's original posting. Or maybe he's just stirring the discussion pot 😳 Surely not

 

As far as I'm aware, 50/50 water glycol under a reliable 1bar cap will not boil until over 125degC. As it is undesirable to run most engines at this temperature, I remain to be convinced that a higher pressure cap is required.

 

The key to that statement is a reliable 1bar cap.

 

Yesterday I suffered a header tank cap failure, which I can confirm is unrelated to head gasket issues.

 

Peter, are you sure? Was it a 'new' cap or could it be one which was previously used on an engine which suffered HGF? I have it from an ex-Rover colleague, who was closely involved with design of the latter MGF project cooling system, that oil contamination of some of the polymer materials (possibly EPDM?) used in the cap can cause degradation and unreliable operation of the pressure cap.

 

It was also indicated that that the shunt connection from the header tank to the bottom hose may be better placed if it were connected into the heater/bypass hose rather than the bottom radiator hose, presumably to assist with air locking and opening the thermostat reliably from first fill.

 

I feel a full application review of the K-series Caterham cooling circuit with my colleague is required the very near future........

 

Ian

 

Green and Silver Roadsport 😬 or Mrs Locust's Blue Roadsport Academy 😬

[Colin Mill]

The key to that statement is a reliable 1bar cap.

 

 

I think the problem we have with the standard system with the header filled to the standard depth is that damage may be done before the pressure cap comes into the equation.

 

The air space in the header is so big that a significant void could build up in the head before the pressure in the system rises to inhibit it. The joggle valve in the head-to-header pipe prevents a large flow between head and header so the fluid in the header is going to be cooler than the outflow from the head - much cooler in my experience. This makes sure that the vapour pressure in the header is going to be significantly lower than the vapour pressure at the hottest points in the head. Since the compression of the air in the header is negligible unless you overfill the header we have a recipe for vapour-filled voids standing the coolant off from the hot points of the cooling ways in the head.

 

I feel that the current system is only safe if the temperature at hottest points in the head are below the un-pressurised boiling point of the coolant. Peter's measurements in his original post pretty much prove this: he said :-

 

"The pressure drops close to 0 bar and sometimes shows negative relative pressure after the thermostat opens"

 

Exactly not what you want with a fully warmed up engine!!

 

My temporary solution - by no means ideal - is to overfill the header so that:-

a) water expansion causes significant compression of the air in the header causing a quicker initial pressure rise that does not rely on the header becoming hot.

b) should localised boiling occur in the head the further compression of the airspace in the header rises the pressure more rapidly than it would with the normal level in the header thus helping limit the volume of the voids in the head.

 

Edited by - Colin Mill on 2 Jun 2008 22:47:54

[Mr Locust]

But why the concentration on generating a high static pressure in the system? Does the water pump not deliver it's flow direct to the block and head creating the highest reliable flow (pressure) in the head anyway? Unless I am mistaken it is coolant flow, not pressure which is the key to heat transfer here.....................

 

Re-reading Peter's original post, the observed behaviour appears in part to support my colleague's comment that the expansion bottle shunt connection could well be better plumbed into the bypass than the bottom hose.

 

From a hot start...

... the system is already heat soaked and is at considerable pressure.

... but the thermostat is closed because the bulk temperature is below the thermostat opening temperature

... the period between the engine starting and thermostat opening is extremely sensitive to engine revs causing pressure increases

... this is when my system pressure spiked to above 1 bar and caused the header tank cap to fail (engine revs of 4000 rpm)

---

 

If the thermostat is shut, assuming high engine speed and zero flow past the thermostat (ignoring the bypass flow for now), the pressure from the delivery side of the pump, into the head, expansion tank, top hose radiator and bottom hose right up to the thermostat will all be at the maximum delivery pressure of the pump. As has been observed, any static system pressure following hot shutdown will add to the pump static head causing the condition where the absolute pressure in the tank is sufficient to vent the cap to atmosphere.

 

Yes, I know this simplifies things by ignoring the bypass flow but this could be regarded as a minimal, deliberate bleed of pump flow to accurately reflect head outlet temperature at the thermostat bulb or as controlled leakage from pump outlet to pump inlet. The pressure will vary quite considerably in the restrictive bypass hose as the pump delivery pressure is at one end and the pump suction at the other although some indication of overall system pressurisation can be drawn at constant rpm conditions.

 

If the tank were shunted onto the bypass hose, effectively in parallel with the bypass, all thermostat conditions will generate a condition where the tank sees pump suction depression at the 5/8" base spigot and pump delivery pressure at the 1/4" top spigot. Therefore the net pressure in the tank due to the pump delivery is zero, balanced, nil nothing. So the cap just sees the system pressure without the water pump head. As the system did not boil uncontrollably following hot shutdown, it could be assumed that the pressure was sufficient.

 

In this scenario the expansion tank could be regarded as an aid to filling, priming and anti cavitation of the water pump. The air gap is necessary to allow for expansion in the event of after boiling in the engine following hot shutdown. Nothing more.

 

How about that for a theory then?

 

Ian

 

Green and Silver Roadsport 😬 or Mrs Locust's Blue Roadsport Academy 😬

 

Edited by - Mr Locust on 3 Jun 2008 20:11:40

[Colin Mill]

But why the concentration on generating a high static pressure in the system?

Because this is the only way of ensuring a reliable increase in boiling point of the fluid. Relying on the pump to provide a pressure in the head is fraught with problems. Firstly, the pump is only capable of providing a pressure differential within the system and so the pump pressure is going to be divided between a pressure increase on the outlet side of the pump and a pressure reduction on the input side.

 

Secondly, the pressure generated by the pump is highly speed dependent. I calculated that the stall pressure of the pump will be about 0.7 bar at 4000 rpm but at 2000 rpm this will be only 0.18 bar - simply too small to significantly affect the boiling point of the fluid even if it was entirely available as a pressure rise in the head which with the bypass flow it won't be. (BTW the bypass has 25% of the cross-sectional area of the radiator hoses and, given the restriction of the radiator itself, the bypass flow is likely to be 20% of the total flow even with the stat fully open and generally more than this so it isn't negligible)

 

A high system pressure is also useful in inhibiting pump cavitation. Remember that it is the absolute pressure of the water at the intake of the pump that determines the onset of cavitation. Once the absolute pressure at the intake of the pump falls to the vapour pressure of the fluid cavitation is very likely. And once you have cavitation you loose significant flow... bye-bye one heat-treated ali head. And to get a handle on this if the intake side of the pump is 0.3bar below ambient (i.e. 0.7bar absolute) then this will happen with the coolant 10 degrees below its normal boiling point!

 

So perhaps I should turn your question around - given all the advantages of an elevated absolute system pressure why would you want anything else? 😬

 

Edited by - Colin Mill on 4 Jun 2008 08:38:37

[Mr Locust]

It is generally nice to have positive pressure at a pump inlet - but not essential.

 

Some engine manufacturers request verification of pressures around the coolant circuit but I have yet to see positive pressure at coolant pump inlet in the required criteria for approval in an OEM installation.

 

The reduced pressure at the pump inlet is a given, but the lower pressure will coincide with a lower temperature of the coolant. Do you know for sure that the pump is suffering from cavitation? Do you have temperature and pressure data at the pump inlet?

 

It is really down to you whether you want to fully investigate, and can justify, the complexity of, adding an external pressurisation system rather than sorting out a group of conventional off the shelf components to deliver the required performance.

 

If you are following the increased pressure idea, have you considered the hose joint integrity, water pump sealing, radiator, heater and head gasket design pressures in the coolant circuit? Also bear in mind whether the pressure cap will overshoot its nominal rated pressure before venting fully.

 

The standard Caterham crimped aluminium header tank radiator has certainly been quite limited in my own experience having had two leak at the header tank seal - one of which was brand new, out of the box, before SVA.

 

Ian

 

Green and Silver Roadsport 😬 or Mrs Locust's Blue Roadsport Academy 😬

[Johnty Lyons]

The air space in the header is so big that a significant void could build up in the head before the pressure in the system rises to inhibit it. The joggle valve in the head-to-header pipe prevents a large flow between head and header so the fluid in the header is going to be cooler than the outflow from the head - much cooler in my experience. This makes sure that the vapour pressure in the header is going to be significantly lower than the vapour pressure at the hottest points in the head.

Guys I take you back to my earlier posting This effect can be completly removed by Replacing the toggle valve with a plumbing outlet spigot and allowing the hottest water in the head to constantly fill the header tank keeping header tank temps same as head temps.

 

---

jj

N.I. L7C AR 🙆🏻

Membership No.3927.

240BHP 1900cc K Series 40th Anniversary

 

[Colin Mill]

If you are following the increased pressure idea, have you considered the hose joint integrity, water pump sealing, radiator, heater and head gasket design pressures in the coolant circuit? Also bear in mind whether the pressure cap will overshoot its nominal rated pressure before venting fully.

 

The standard Caterham crimped aluminium header tank radiator has certainly been quite limited in my own experience having had two leak at the header tank seal - one of which was brand new, out of the box, before SVA.

---

 

Lets consider the stresses on the hoses and radiator at 3Bar (3 E5 Pascal) – enough for a 1Bar system allowing for a 50% overshoot and a safety factor of 2.

 

The hoses are 32mm diameter.

 

Consider first the circumferential tension trying to split the hose down its length:-

 

Circumferential tension per unit length = Pressure x Radius = 3 E5Pascals x 16 E-3m = 4,800N/m

 

Now consider the longitudinal tension:-

 

Longitudinal tension = Pressure x cross sectional area / circumference = Pressure x diameter/4

= (3 E5 x 32 E-3) / 4 = 9600/4 = 2,400 N/m (note that its half the circumferential tension as mentioned earlier)

 

The vector sum of these forces = 5,370 N/m

 

Assuming we use polypropylene reinforcement with a tensile strength of 35MPa the reinforcement layer needs to have a thickness of 5370 / 35E6 metres = 0.15mm.

 

So it is not much of a problem to make a hose to withstand the stress of 1Bar with a safety factor of 3.

 

Consider now the stress on the top and bottom tanks of the radiator:-

 

Taking the plan area of the top tank as 60mm x 400mm we have an area of 0.024 sq.m and, at 3Bar a force of 0.024 x 3 E5 N = 7200N trying to pop it off. This stress is carried by the periphery of the tank with a length of 920mm so the tension per unit length = 7,800N/m

 

Even soft aluminium with a tensile strength of only 80MPa only needs a wall thickness of 0.1mm to withstand this! If the Caterham radiators leaked on you it can hardly be to excessive stress – just poor manufacture.

 

We could go on but the bottom line is that the pressurisation to 1 Bar doesn't introduce a massive stress into any component. And of course none of this is an automotive novelty. The 'airless' system with a 13psi stant cap and overflow bottle was what you used to find under just about every bonnet (and hood) in the World – a system that ensures early pressurisation of the system to the cap rating. If the K series had a wonderful reliability record then I would accept that its cooling system design was fine but the truth is it has an absolutely stinking reputation and it is not at all clear if the HGF problems are not at least sometimes a result of the cooling system. From the number of modifications Rover and Land Rover tried on the system there is clearly a significant doubt in that quarter as to the ideality of the system!

 

 

Edited by - Colin Mill on 5 Jun 2008 09:57:37

[Mr Locust]

Colin,

 

enough calculations!! You have convinced yourself that you need something different so you need to satisfy your own curiosity.

 

For engine coolant circuits in the day job, I almost always specify a top hose, bottom hose with a drain spigot connection and a 1bar plunger type pressure cap with a secondary seal fitted to a two stage neck on the top tank of the radiator and an unpressurised 'dip tube' recovery bottle....................No external pressurisation though

 

Johnty,

 

spot on, I believe even Rover deleted it in the end.

 

Having discussed this further with my colleague today, his recollection is that the Caterham installation replicates that seen on the Metro, R214, R25 etc which was apparently a pain to fill and de-aerate reliably in production (let alone the aftermarket), right up to the demise of Rover. He was also quite careful to distance himself from the original installation which was in place before he joined the company! This would of course fit in with when Caterham started to use the engine.

 

For minimal effort he believes a minor modification to the Caterham install would be beneficial - replicating the later MGF installation. By moving the expansion bottle shunt connection from the bottom hose submarine pipe over to a tee into the bypass hose, as close as possible to the spigot on the back of the thermostat housing. More reliable filling, de-aeration and more stable temperature/pressure condition in the expansion bottle (and water pump inlet) at the complete range of thermostat operating conditions are likely benefits. All, of course, totally untested by myself but hopefully relevant to this discussion. Alternatively you could go to a bypass sealing remote thermostat, PRRT..........blah, blah, blah. Just depends how much time effort and money you want to put into your solution.

 

Peter,

 

if you did consider the above to be worth trying I would genuinely be interested in difference it makes to your observed pressure.

 

Ian

 

Green and Silver Roadsport 😬 or Mrs Locust's Blue Roadsport Academy 😬

[Colin Mill]

enough calculations!! You have convinced yourself that you need something different so you need to satisfy your own curiosity.

I'm sorry if you find the calculations OTT but I find far too many engineers in the UK simply don't do the sums (unlike their German counterparts).

 

My aim is to explain why I think we need something different to the system as it is currently installed in the K powered Caterham. I would personally be happy if the car had been fitted with a 'conventional' cooling system - I get the impression you would agree...

 

For engine coolant circuits in the day job, I almost always specify a top hose, bottom hose with a drain spigot connection and a 1bar plunger type pressure cap with a secondary seal fitted to a two stage neck on the top tank of the radiator and an unpressurised 'dip tube' recovery bottle.

 

I agree that removing the joggle valve and re-plumbing the expansion tank tail to the bypass would be a significant improvement to the existing system by allowing the expansion tank to more or less track the head temperature independent of the thermostat operation. Perhaps this will be enough though the Rover cap remains in my mind an unknown quantity.

[Johnty Lyons]

Just to give you some more background My car is as you all know a 240BHP 1900cc k it has ever since it had it's first minor old head port by DVA,and the original beta Emerald.. the big 4mm ID plumbing spigot in the inlet manifold through which there is a constant stream of head temp water. The car had done 12K miles including extreme temp continental touring [2500 miles trans USA inc where there where days when the air temp never dropped below 100 deg] and numerous Track days. it is still on its original Rover Cap has never blown a head gasket and never overheated. So there can't be that much wrong. I use SPA twin digital gauges to monitor temps and pressures Not quite as sophisticated as Peter but they seem to work for me. 😶‍🌫️

 

---

jj

N.I. L7C AR 🙆🏻

Membership No.3927.

240BHP 1900cc K Series 40th Anniversary